Review fixes for advancing series and provider occurrences (#135, #137) (#147)

In Agendula's own storage, ticking off the current occurrence of a repeating task now moves the series on to its next date and keeps a completed copy, as tasks.org and Apple Reminders do.

Before, the series stayed open and the completion was a `RECURRENCE-ID` override with `STATUS:COMPLETED` in the same resource. That's valid iCalendar, but Infomaniak's web view and Apple Reminders ignore it and keep showing the task as open, which is what #137 reports. The reporter hasn't confirmed their storage mode yet. With tasks.org/OpenTasks the provider already moves the series on, so this change is about the built-in CalDAV sync.

How it works (`RoomTasksDataSource`):
- Ticking or cancelling the series' **first open occurrence** moves the master to its next open occurrence. Closed overrides on either side become standalone completed copies (new UID, fresh alarm identities), including the ones 1.3.0 left behind. `COUNT` drops by the occurrences used up (counted before `EXDATE`, shared with `splitSeries`), `EXDATE` values before the new start are dropped, and status and progress are reset. An edited occurrence is converted in place, so its id, edits and own properties survive. Lines inside nested components, such as a `VALARM`'s `UID`, are kept.
- Series with `RDATE`s keep the override model, because moving the anchor onto an `RDATE` would re-align the `RRULE`. So do recurring subtasks, whose copies would otherwise pile up under the parent.
- How the series moved is recorded in two new local tables, `advanced_occurrences` and `advanced_detached` (schema v5, migration and migration test included). They are never part of the synced `VTODO`. A record keeps mapping the occurrence to its copy (pruned after 30 days), so the detail screen and reminders find the copy even after it was edited. Every save path goes through one check: when the only thing that changes on a copy is that it reopens (compared as the editor writes values back), the move is undone. That covers undo, the completed section, the editor, the detail screen and other apps through Agendula's provider. The anchor goes back, `COUNT` regains what it spent, the dropped `EXDATE`s return in the list's own value shape, other edits made since are kept, and a reopened state such as 40% stays on the occurrence. Detached overrides come back as overrides; one deleted since, or given its own parent or subtasks, is excluded instead. A copy that was edited, moved or given subtasks, or whose series moved on, changed its rule or was deleted, reopens as a task of its own.
- If a sync discards the local version of a moved series because the server's is newer, the copies of moves the server never saw are deleted too, and the server's own overrides stand for those occurrences. Moves already uploaded are marked settled and left alone.
- `task(series, anchor)` resolves a moved-past occurrence to its copy, so the detail screen and reminder checks see it as done. The repository then acts on the row it resolved, including the detail screen's subtasks, so deleting or saving from a screen opened before the move can't hit the whole series.
- A stale tick (an old notification or widget row) for an occurrence the series has already moved past does nothing.
- Ticking the last open occurrence closes the series, and reopening any of its occurrences reopens the series.
- Any other occurrence (with earlier ones still open) keeps the override model.

Also fixed here: `AgendulaTaskProviderTest` failed on `release/v1.3.1` whenever Agendula ran on an external provider, because `ProviderWriter` checked the active store first. Read-only tables are now refused as such whichever store is active (one exhaustive `when` per operation), and the two tests that check value errors skip when the provider takes no writes at all.

Tests: `RoomTasksDataSourceTest` covers each case above. There is also a migration test, a `ProviderWriterTest` for reopening through the provider, and `RoomSyncStoreAdvancesTest` (full flavour) for the sync discard. The full on-device suite (223 tests) passes on the emulator, and the unit tests pass for both flavours.

Closes #137

Co-authored-by: Jean-Luc Makiola <business@jeanlucmakiola.de>
Reviewed-on: https://codeberg.org/jlmakiola/agendula/pulls/144

CHANGELOG.md

@@ -7,6 +7,24 @@ All notable changes to this project are documented here. The format follows
 
 ## [Unreleased]
 
+## [1.3.1] - 2026-11-09
+
+### Changed
+- Ticking off the current occurrence of a repeating task moves the series on
+  to its next date and keeps a completed copy, as tasks.org and Apple
+  Reminders do. Other apps and the web views of servers such as Infomaniak now
+  see it as done instead of still open. Unticking it moves the series back,
+  as long as the series hasn't moved on or changed its start since (#137).
+
+### Fixed
+- With tasks.org as the storage provider, recurring tasks are recognised as
+  recurring. Before, editing one removed its recurrence and ticking it off
+  closed the whole series. With tasks.org and OpenTasks alike, the editor now
+  shows the task's repeat rule instead of none, and an occurrence changed on
+  its own stays part of its series (#135).
+- With tasks.org or OpenTasks as the storage provider, the order you give your
+  lists is kept instead of snapping back (#136).
+
 ## [1.3.0] - 2026-11-02
 
 ### Added

app/build.gradle.kts

@@ -33,8 +33,8 @@ android {
         // (1.0.x: 1.0.0 -> 10000; from 1.1.0: 1.1.0-beta.1 -> 1010001,
         // 1.1.0 -> 1010099), and CI fails if the committed one doesn't match.
         // See docs/RELEASING.md.
-        versionCode = 1030099
-        versionName = "1.3.0"
+        versionCode = 1030199
+        versionName = "1.3.1"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
 

app/schemas/de.jeanlucmakiola.agendula.data.tasks.room.TasksDatabase/5.json

@@ -0,0 +1,768 @@
+{
+  "formatVersion": 1,
+  "database": {
+    "version": 5,
+    "identityHash": "fcb14ff68d892c72fa7459204276abb6",
+    "entities": [
+      {
+        "tableName": "accounts",
+        "createSql": "CREATE TABLE IF NOT EXISTS `${TABLE_NAME}` (`id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `display_name` TEXT NOT NULL, `principal_url` TEXT, `home_set_url` TEXT, `username` TEXT, `last_sync_at` INTEGER, `last_sync_error` TEXT, `label` TEXT)",
+        "fields": [
+          {
+            "fieldPath": "id",
+            "columnName": "id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "displayName",
+            "columnName": "display_name",
+            "affinity": "TEXT",
+            "notNull": true
+          },
+          {
+            "fieldPath": "principalUrl",
+            "columnName": "principal_url",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "homeSetUrl",
+            "columnName": "home_set_url",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "username",
+            "columnName": "username",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "lastSyncAt",
+            "columnName": "last_sync_at",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "lastSyncError",
+            "columnName": "last_sync_error",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "label",
+            "columnName": "label",
+            "affinity": "TEXT"
+          }
+        ],
+        "primaryKey": {
+          "autoGenerate": true,
+          "columnNames": [
+            "id"
+          ]
+        }
+      },
+      {
+        "tableName": "task_lists",
+        "createSql": "CREATE TABLE IF NOT EXISTS `${TABLE_NAME}` (`id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `name` TEXT NOT NULL, `color` INTEGER NOT NULL, `account_id` INTEGER, `is_visible` INTEGER NOT NULL DEFAULT 1, `is_synced` INTEGER NOT NULL DEFAULT 1, `owner` TEXT, `is_read_only` INTEGER NOT NULL DEFAULT 0, `sort_order` INTEGER NOT NULL DEFAULT 0, `href` TEXT, `ctag` TEXT, `sync_token` TEXT, `is_dirty` INTEGER NOT NULL DEFAULT 0, FOREIGN KEY(`account_id`) REFERENCES `accounts`(`id`) ON UPDATE NO ACTION ON DELETE SET NULL )",
+        "fields": [
+          {
+            "fieldPath": "id",
+            "columnName": "id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "name",
+            "columnName": "name",
+            "affinity": "TEXT",
+            "notNull": true
+          },
+          {
+            "fieldPath": "color",
+            "columnName": "color",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "accountId",
+            "columnName": "account_id",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "isVisible",
+            "columnName": "is_visible",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "1"
+          },
+          {
+            "fieldPath": "isSynced",
+            "columnName": "is_synced",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "1"
+          },
+          {
+            "fieldPath": "owner",
+            "columnName": "owner",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "isReadOnly",
+            "columnName": "is_read_only",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          },
+          {
+            "fieldPath": "sortOrder",
+            "columnName": "sort_order",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          },
+          {
+            "fieldPath": "href",
+            "columnName": "href",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "ctag",
+            "columnName": "ctag",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "syncToken",
+            "columnName": "sync_token",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "isDirty",
+            "columnName": "is_dirty",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          }
+        ],
+        "primaryKey": {
+          "autoGenerate": true,
+          "columnNames": [
+            "id"
+          ]
+        },
+        "indices": [
+          {
+            "name": "index_task_lists_account_id",
+            "unique": false,
+            "columnNames": [
+              "account_id"
+            ],
+            "orders": [],
+            "createSql": "CREATE INDEX IF NOT EXISTS `index_task_lists_account_id` ON `${TABLE_NAME}` (`account_id`)"
+          }
+        ],
+        "foreignKeys": [
+          {
+            "table": "accounts",
+            "onDelete": "SET NULL",
+            "onUpdate": "NO ACTION",
+            "columns": [
+              "account_id"
+            ],
+            "referencedColumns": [
+              "id"
+            ]
+          }
+        ]
+      },
+      {
+        "tableName": "tasks",
+        "createSql": "CREATE TABLE IF NOT EXISTS `${TABLE_NAME}` (`id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `list_id` INTEGER NOT NULL, `uid` TEXT NOT NULL, `href` TEXT, `etag` TEXT, `title` TEXT, `description` TEXT, `location` TEXT, `url` TEXT, `color` INTEGER, `categories` TEXT NOT NULL DEFAULT '', `status` INTEGER NOT NULL DEFAULT 0, `percent_complete` INTEGER, `completed_at` INTEGER, `priority` INTEGER NOT NULL DEFAULT 0, `classification` INTEGER, `dtstart` INTEGER, `due` INTEGER, `duration` TEXT, `is_all_day` INTEGER NOT NULL DEFAULT 0, `timezone` TEXT, `rrule` TEXT, `rdate` TEXT, `exdate` TEXT, `recurrence_id` INTEGER, `master_id` INTEGER, `parent_id` INTEGER, `sort_order` INTEGER NOT NULL DEFAULT 0, `created_at` INTEGER, `last_modified` INTEGER, `sequence` INTEGER NOT NULL DEFAULT 0, `is_dirty` INTEGER NOT NULL DEFAULT 0, `is_deleted` INTEGER NOT NULL DEFAULT 0, `unknown_properties` TEXT, `reminders_off` INTEGER NOT NULL DEFAULT 0, FOREIGN KEY(`list_id`) REFERENCES `task_lists`(`id`) ON UPDATE NO ACTION ON DELETE CASCADE , FOREIGN KEY(`master_id`) REFERENCES `tasks`(`id`) ON UPDATE NO ACTION ON DELETE CASCADE , FOREIGN KEY(`parent_id`) REFERENCES `tasks`(`id`) ON UPDATE NO ACTION ON DELETE SET NULL )",
+        "fields": [
+          {
+            "fieldPath": "id",
+            "columnName": "id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "listId",
+            "columnName": "list_id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "uid",
+            "columnName": "uid",
+            "affinity": "TEXT",
+            "notNull": true
+          },
+          {
+            "fieldPath": "href",
+            "columnName": "href",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "etag",
+            "columnName": "etag",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "title",
+            "columnName": "title",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "description",
+            "columnName": "description",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "location",
+            "columnName": "location",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "url",
+            "columnName": "url",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "color",
+            "columnName": "color",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "categories",
+            "columnName": "categories",
+            "affinity": "TEXT",
+            "notNull": true,
+            "defaultValue": "''"
+          },
+          {
+            "fieldPath": "status",
+            "columnName": "status",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          },
+          {
+            "fieldPath": "percentComplete",
+            "columnName": "percent_complete",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "completedAt",
+            "columnName": "completed_at",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "priority",
+            "columnName": "priority",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          },
+          {
+            "fieldPath": "classification",
+            "columnName": "classification",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "dtstart",
+            "columnName": "dtstart",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "due",
+            "columnName": "due",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "duration",
+            "columnName": "duration",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "isAllDay",
+            "columnName": "is_all_day",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          },
+          {
+            "fieldPath": "timezone",
+            "columnName": "timezone",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "rrule",
+            "columnName": "rrule",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "rdate",
+            "columnName": "rdate",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "exdate",
+            "columnName": "exdate",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "recurrenceId",
+            "columnName": "recurrence_id",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "masterId",
+            "columnName": "master_id",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "parentId",
+            "columnName": "parent_id",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "sortOrder",
+            "columnName": "sort_order",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          },
+          {
+            "fieldPath": "createdAt",
+            "columnName": "created_at",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "lastModified",
+            "columnName": "last_modified",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "sequence",
+            "columnName": "sequence",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          },
+          {
+            "fieldPath": "isDirty",
+            "columnName": "is_dirty",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          },
+          {
+            "fieldPath": "isDeleted",
+            "columnName": "is_deleted",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          },
+          {
+            "fieldPath": "unknownProperties",
+            "columnName": "unknown_properties",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "remindersOff",
+            "columnName": "reminders_off",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          }
+        ],
+        "primaryKey": {
+          "autoGenerate": true,
+          "columnNames": [
+            "id"
+          ]
+        },
+        "indices": [
+          {
+            "name": "index_tasks_list_id_is_deleted",
+            "unique": false,
+            "columnNames": [
+              "list_id",
+              "is_deleted"
+            ],
+            "orders": [],
+            "createSql": "CREATE INDEX IF NOT EXISTS `index_tasks_list_id_is_deleted` ON `${TABLE_NAME}` (`list_id`, `is_deleted`)"
+          },
+          {
+            "name": "index_tasks_parent_id",
+            "unique": false,
+            "columnNames": [
+              "parent_id"
+            ],
+            "orders": [],
+            "createSql": "CREATE INDEX IF NOT EXISTS `index_tasks_parent_id` ON `${TABLE_NAME}` (`parent_id`)"
+          },
+          {
+            "name": "index_tasks_master_id_recurrence_id",
+            "unique": false,
+            "columnNames": [
+              "master_id",
+              "recurrence_id"
+            ],
+            "orders": [],
+            "createSql": "CREATE INDEX IF NOT EXISTS `index_tasks_master_id_recurrence_id` ON `${TABLE_NAME}` (`master_id`, `recurrence_id`)"
+          },
+          {
+            "name": "index_tasks_is_dirty",
+            "unique": false,
+            "columnNames": [
+              "is_dirty"
+            ],
+            "orders": [],
+            "createSql": "CREATE INDEX IF NOT EXISTS `index_tasks_is_dirty` ON `${TABLE_NAME}` (`is_dirty`)"
+          },
+          {
+            "name": "index_tasks_list_id_uid_recurrence_id",
+            "unique": true,
+            "columnNames": [
+              "list_id",
+              "uid",
+              "recurrence_id"
+            ],
+            "orders": [],
+            "createSql": "CREATE UNIQUE INDEX IF NOT EXISTS `index_tasks_list_id_uid_recurrence_id` ON `${TABLE_NAME}` (`list_id`, `uid`, `recurrence_id`)"
+          }
+        ],
+        "foreignKeys": [
+          {
+            "table": "task_lists",
+            "onDelete": "CASCADE",
+            "onUpdate": "NO ACTION",
+            "columns": [
+              "list_id"
+            ],
+            "referencedColumns": [
+              "id"
+            ]
+          },
+          {
+            "table": "tasks",
+            "onDelete": "CASCADE",
+            "onUpdate": "NO ACTION",
+            "columns": [
+              "master_id"
+            ],
+            "referencedColumns": [
+              "id"
+            ]
+          },
+          {
+            "table": "tasks",
+            "onDelete": "SET NULL",
+            "onUpdate": "NO ACTION",
+            "columns": [
+              "parent_id"
+            ],
+            "referencedColumns": [
+              "id"
+            ]
+          }
+        ]
+      },
+      {
+        "tableName": "task_alarms",
+        "createSql": "CREATE TABLE IF NOT EXISTS `${TABLE_NAME}` (`id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `task_id` INTEGER NOT NULL, `minutes_before` INTEGER NOT NULL, `reference` TEXT NOT NULL DEFAULT 'DUE', `message` TEXT, `uid` TEXT NOT NULL DEFAULT '', `raw` TEXT, FOREIGN KEY(`task_id`) REFERENCES `tasks`(`id`) ON UPDATE NO ACTION ON DELETE CASCADE )",
+        "fields": [
+          {
+            "fieldPath": "id",
+            "columnName": "id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "taskId",
+            "columnName": "task_id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "minutesBefore",
+            "columnName": "minutes_before",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "reference",
+            "columnName": "reference",
+            "affinity": "TEXT",
+            "notNull": true,
+            "defaultValue": "'DUE'"
+          },
+          {
+            "fieldPath": "message",
+            "columnName": "message",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "uid",
+            "columnName": "uid",
+            "affinity": "TEXT",
+            "notNull": true,
+            "defaultValue": "''"
+          },
+          {
+            "fieldPath": "raw",
+            "columnName": "raw",
+            "affinity": "TEXT"
+          }
+        ],
+        "primaryKey": {
+          "autoGenerate": true,
+          "columnNames": [
+            "id"
+          ]
+        },
+        "indices": [
+          {
+            "name": "index_task_alarms_task_id",
+            "unique": false,
+            "columnNames": [
+              "task_id"
+            ],
+            "orders": [],
+            "createSql": "CREATE INDEX IF NOT EXISTS `index_task_alarms_task_id` ON `${TABLE_NAME}` (`task_id`)"
+          }
+        ],
+        "foreignKeys": [
+          {
+            "table": "tasks",
+            "onDelete": "CASCADE",
+            "onUpdate": "NO ACTION",
+            "columns": [
+              "task_id"
+            ],
+            "referencedColumns": [
+              "id"
+            ]
+          }
+        ]
+      },
+      {
+        "tableName": "advanced_occurrences",
+        "createSql": "CREATE TABLE IF NOT EXISTS `${TABLE_NAME}` (`copy_id` INTEGER NOT NULL, `master_id` INTEGER NOT NULL, `anchor` INTEGER NOT NULL, `advanced_to` INTEGER NOT NULL, `prior_dtstart` INTEGER, `prior_due` INTEGER, `prior_status` INTEGER NOT NULL, `prior_percent_complete` INTEGER, `prior_completed_at` INTEGER, `spent` INTEGER NOT NULL, `rule_after` TEXT, `dropped_exdates` TEXT NOT NULL DEFAULT '', `from_override` INTEGER NOT NULL, `undoable` INTEGER NOT NULL DEFAULT 1, `settled` INTEGER NOT NULL DEFAULT 0, PRIMARY KEY(`copy_id`), FOREIGN KEY(`copy_id`) REFERENCES `tasks`(`id`) ON UPDATE NO ACTION ON DELETE CASCADE , FOREIGN KEY(`master_id`) REFERENCES `tasks`(`id`) ON UPDATE NO ACTION ON DELETE CASCADE )",
+        "fields": [
+          {
+            "fieldPath": "copyId",
+            "columnName": "copy_id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "masterId",
+            "columnName": "master_id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "anchor",
+            "columnName": "anchor",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "advancedTo",
+            "columnName": "advanced_to",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "priorDtstart",
+            "columnName": "prior_dtstart",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "priorDue",
+            "columnName": "prior_due",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "priorStatus",
+            "columnName": "prior_status",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "priorPercentComplete",
+            "columnName": "prior_percent_complete",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "priorCompletedAt",
+            "columnName": "prior_completed_at",
+            "affinity": "INTEGER"
+          },
+          {
+            "fieldPath": "spent",
+            "columnName": "spent",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "ruleAfter",
+            "columnName": "rule_after",
+            "affinity": "TEXT"
+          },
+          {
+            "fieldPath": "droppedExdates",
+            "columnName": "dropped_exdates",
+            "affinity": "TEXT",
+            "notNull": true,
+            "defaultValue": "''"
+          },
+          {
+            "fieldPath": "fromOverride",
+            "columnName": "from_override",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "undoable",
+            "columnName": "undoable",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "1"
+          },
+          {
+            "fieldPath": "settled",
+            "columnName": "settled",
+            "affinity": "INTEGER",
+            "notNull": true,
+            "defaultValue": "0"
+          }
+        ],
+        "primaryKey": {
+          "autoGenerate": false,
+          "columnNames": [
+            "copy_id"
+          ]
+        },
+        "indices": [
+          {
+            "name": "index_advanced_occurrences_master_id_anchor",
+            "unique": false,
+            "columnNames": [
+              "master_id",
+              "anchor"
+            ],
+            "orders": [],
+            "createSql": "CREATE INDEX IF NOT EXISTS `index_advanced_occurrences_master_id_anchor` ON `${TABLE_NAME}` (`master_id`, `anchor`)"
+          }
+        ],
+        "foreignKeys": [
+          {
+            "table": "tasks",
+            "onDelete": "CASCADE",
+            "onUpdate": "NO ACTION",
+            "columns": [
+              "copy_id"
+            ],
+            "referencedColumns": [
+              "id"
+            ]
+          },
+          {
+            "table": "tasks",
+            "onDelete": "CASCADE",
+            "onUpdate": "NO ACTION",
+            "columns": [
+              "master_id"
+            ],
+            "referencedColumns": [
+              "id"
+            ]
+          }
+        ]
+      },
+      {
+        "tableName": "advanced_detached",
+        "createSql": "CREATE TABLE IF NOT EXISTS `${TABLE_NAME}` (`id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `copy_id` INTEGER NOT NULL, `anchor` INTEGER NOT NULL, `task_id` INTEGER, FOREIGN KEY(`copy_id`) REFERENCES `advanced_occurrences`(`copy_id`) ON UPDATE NO ACTION ON DELETE CASCADE , FOREIGN KEY(`task_id`) REFERENCES `tasks`(`id`) ON UPDATE NO ACTION ON DELETE SET NULL )",
+        "fields": [
+          {
+            "fieldPath": "id",
+            "columnName": "id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "copyId",
+            "columnName": "copy_id",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "anchor",
+            "columnName": "anchor",
+            "affinity": "INTEGER",
+            "notNull": true
+          },
+          {
+            "fieldPath": "taskId",
+            "columnName": "task_id",
+            "affinity": "INTEGER"
+          }
+        ],
+        "primaryKey": {
+          "autoGenerate": true,
+          "columnNames": [
+            "id"
+          ]
+        },
+        "indices": [
+          {
+            "name": "index_advanced_detached_copy_id",
+            "unique": false,
+            "columnNames": [
+              "copy_id"
+            ],
+            "orders": [],
+            "createSql": "CREATE INDEX IF NOT EXISTS `index_advanced_detached_copy_id` ON `${TABLE_NAME}` (`copy_id`)"
+          },
+          {
+            "name": "index_advanced_detached_task_id",
+            "unique": false,
+            "columnNames": [
+              "task_id"
+            ],
+            "orders": [],
+            "createSql": "CREATE INDEX IF NOT EXISTS `index_advanced_detached_task_id` ON `${TABLE_NAME}` (`task_id`)"
+          }
+        ],
+        "foreignKeys": [
+          {
+            "table": "advanced_occurrences",
+            "onDelete": "CASCADE",
+            "onUpdate": "NO ACTION",
+            "columns": [
+              "copy_id"
+            ],
+            "referencedColumns": [
+              "copy_id"
+            ]
+          },
+          {
+            "table": "tasks",
+            "onDelete": "SET NULL",
+            "onUpdate": "NO ACTION",
+            "columns": [
+              "task_id"
+            ],
+            "referencedColumns": [
+              "id"
+            ]
+          }
+        ]
+      }
+    ],
+    "setupQueries": [
+      "CREATE TABLE IF NOT EXISTS room_master_table (id INTEGER PRIMARY KEY,identity_hash TEXT)",
+      "INSERT OR REPLACE INTO room_master_table (id,identity_hash) VALUES(42, 'fcb14ff68d892c72fa7459204276abb6')"
+    ]
+  }
+}

app/src/androidTest/java/de/jeanlucmakiola/agendula/data/tasks/ExternalProviderListOrderTest.kt

@@ -0,0 +1,76 @@
+package de.jeanlucmakiola.agendula.data.tasks
+
+import android.content.Context
+import androidx.test.core.app.ApplicationProvider
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.google.common.truth.Truth.assertThat
+import org.junit.After
+import org.junit.Assume.assumeTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import java.util.concurrent.CountDownLatch
+import java.util.concurrent.TimeUnit
+
+/**
+ * List order against a real external provider (#136). Skipped unless one is
+ * installed and its permissions are granted.
+ */
+@RunWith(AndroidJUnit4::class)
+class ExternalProviderListOrderTest {
+
+    private val context = ApplicationProvider.getApplicationContext<Context>()
+    private val resolver = ProviderResolver(AndroidProviderEnvironment(context)).apply {
+        storageMode = StorageMode.EXTERNAL
+    }
+    private val source = AndroidTasksDataSource(context, resolver)
+    private val created = mutableListOf<Long>()
+
+    @Before
+    fun setUp() {
+        val provider = resolver.resolve()
+        assumeTrue("no external tasks provider, or not granted", provider != null && resolver.hasPermission(provider))
+        listOf("Order A", "Order B", "Order C").forEach { created += source.createLocalList(it, 0) }
+    }
+
+    @After
+    fun tearDown() = created.forEach { runCatching { source.deleteList(it) } }
+
+    private fun ours() = source.taskLists().map { it.id }.filter { it in created }
+
+    /** Waits until the provider has gone quiet, so a change seen afterwards is the test's own. */
+    private fun settle() {
+        var last = System.currentTimeMillis()
+        val observer = source.registerObserver { last = System.currentTimeMillis() }
+        while (System.currentTimeMillis() - last < 1_500) Thread.sleep(100)
+        observer.close()
+    }
+
+    @Test
+    fun aReorderIsKeptAndReported() {
+        val (a, b, c) = created
+        settle()
+        val changed = CountDownLatch(1)
+        val observer = source.registerObserver { changed.countDown() }
+
+        source.reorderLists(listOf(c, a, b))
+
+        assertThat(changed.await(5, TimeUnit.SECONDS)).isTrue()
+        observer.close()
+        assertThat(ours()).containsExactly(c, a, b).inOrder()
+        assertThat(AndroidTasksDataSource(context, resolver).taskLists().map { it.id }.filter { it in created })
+            .containsExactly(c, a, b).inOrder()
+    }
+
+    @Test
+    fun aDeletedListLeavesNoPositionBehind() {
+        val (a, b, c) = created
+        source.reorderLists(listOf(c, a, b))
+
+        source.deleteList(c)
+        created -= c
+
+        source.taskLists()
+        assertThat(source.storedListOrder().keys).doesNotContain(c)
+    }
+}

app/src/androidTest/java/de/jeanlucmakiola/agendula/data/tasks/ExternalProviderRecurrenceTest.kt

@@ -0,0 +1,361 @@
+package de.jeanlucmakiola.agendula.data.tasks
+
+import android.content.ContentValues
+import android.content.Context
+import androidx.test.core.app.ApplicationProvider
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.google.common.truth.Truth.assertThat
+import de.jeanlucmakiola.agendula.data.tasks.TasksContract.Tasks
+import de.jeanlucmakiola.agendula.domain.Task
+import de.jeanlucmakiola.agendula.domain.TaskForm
+import de.jeanlucmakiola.agendula.domain.TaskReminder
+import org.junit.After
+import org.junit.Assert.assertThrows
+import org.junit.Assume.assumeTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import kotlin.time.Clock
+import kotlin.time.Duration.Companion.hours
+import kotlin.time.Duration.Companion.milliseconds
+import kotlin.time.Instant
+
+/**
+ * Recurrence against a real external provider (#135). Skipped unless one is
+ * installed and its permissions are granted, so it only runs where set up:
+ * `pm grant <app> org.tasks.permission.READ_TASKS` (and WRITE_TASKS).
+ */
+@RunWith(AndroidJUnit4::class)
+class ExternalProviderRecurrenceTest {
+
+    private val context = ApplicationProvider.getApplicationContext<Context>()
+    private val resolver = ProviderResolver(AndroidProviderEnvironment(context)).apply {
+        storageMode = StorageMode.EXTERNAL
+    }
+    private val source = AndroidTasksDataSource(context, resolver)
+    private var listId = 0L
+
+    @Before
+    fun setUp() {
+        val provider = resolver.resolve()
+        assumeTrue("no external tasks provider, or not granted", provider != null && resolver.hasPermission(provider))
+        listId = source.createLocalList("Recurrence test", 0)
+    }
+
+    @After
+    fun tearDown() {
+        if (listId != 0L) source.deleteList(listId)
+    }
+
+    private fun dailyTask(): Long {
+        // Milliseconds and all, as the "start today" default gives.
+        val start = Clock.System.now() + 1.hours
+        return source.insertTask(
+            TaskForm(title = "Daily", listId = listId, start = start, due = start + 1.hours, rrule = "FREQ=DAILY"),
+            reminders = null,
+        )
+    }
+
+    private fun storedRule(taskId: Long): String? = context.contentResolver.query(
+        TasksContract.tasksUri(resolver.resolve()!!.authority),
+        arrayOf(Tasks.RRULE),
+        "${Tasks.ID} = ?",
+        arrayOf(taskId.toString()),
+        null,
+    )?.use { c -> if (c.moveToFirst()) c.getString(0) else null }
+
+    private fun setRule(taskId: Long, rule: String) {
+        context.contentResolver.update(
+            TasksContract.tasksUri(resolver.resolve()!!.authority),
+            ContentValues().apply { put(Tasks.RRULE, rule) },
+            "${Tasks.ID} = ?",
+            arrayOf(taskId.toString()),
+        )
+    }
+
+    @Test
+    fun aSeriesReadsAsRecurringWithItsRule() {
+        val id = dailyTask()
+
+        val listed = source.tasks(TaskQuery(listId = listId)).filter { it.taskId == id }.minBy { it.occurrenceStart!! }
+        assertThat(listed.isRecurring).isTrue()
+        assertThat(listed.isOccurrence).isTrue()
+        assertThat(listed.recurrenceRule).isEqualTo("FREQ=DAILY")
+
+        val opened = source.task(id)!!
+        assertThat(opened.recurrenceRule).isEqualTo("FREQ=DAILY")
+    }
+
+    @Test
+    fun savingTheSeriesKeepsItsRule() {
+        val id = dailyTask()
+        val task = source.task(id)!!
+
+        source.updateSeries(
+            id,
+            task.occurrenceStart!!,
+            TaskForm(title = "Daily, renamed", listId = listId, start = task.start, due = task.due, rrule = task.recurrenceRule),
+            reminders = null,
+        )
+
+        assertThat(storedRule(id)).isEqualTo("FREQ=DAILY")
+    }
+
+    @Test
+    fun completingAnOccurrenceMovesTheSeriesOn() {
+        val id = dailyTask()
+        val task = source.task(id)!!
+
+        source.setCompletedInstance(id, task.occurrenceStart!!, completed = true)
+
+        assertSeriesMovedOn(id, task.occurrenceStart!!)
+    }
+
+    @Test
+    fun aTickedOccurrenceReadsAsItsCopy() {
+        val id = dailyTask()
+        val anchor = source.task(id)!!.occurrenceStart!!
+
+        source.setCompletedInstance(id, anchor, completed = true)
+
+        val read = source.task(id, anchor)!!
+        assertThat(read.taskId).isNotEqualTo(id)
+        assertThat(read.isClosed).isTrue()
+    }
+
+    @Test
+    fun undoingATickMovesTheSeriesBack() {
+        val id = dailyTask()
+        val anchor = source.task(id)!!.occurrenceStart!!
+        source.setCompletedInstance(id, anchor, completed = true)
+        val copy = source.task(id, anchor)!!.taskId
+
+        source.setCompletedInstance(id, anchor, completed = false)
+
+        val read = source.task(id, anchor)!!
+        assertThat(read.taskId).isEqualTo(id)
+        assertThat(read.isClosed).isFalse()
+        assertThat(source.task(copy)).isNull()
+    }
+
+    @Test
+    fun reopeningTheCopyItselfMovesTheSeriesBack() {
+        val id = dailyTask()
+        val anchor = source.task(id)!!.occurrenceStart!!
+        source.setCompletedInstance(id, anchor, completed = true)
+        val copy = source.task(id, anchor)!!.taskId
+
+        source.setCompleted(copy, completed = false)
+
+        assertThat(source.task(id, anchor)!!.taskId).isEqualTo(id)
+        assertThat(source.task(copy)).isNull()
+    }
+
+    @Test
+    fun aSeriesChangedSinceTheTickReopensOnlyTheCopy() {
+        val id = dailyTask()
+        val anchor = source.task(id)!!.occurrenceStart!!
+        source.setCompletedInstance(id, anchor, completed = true)
+        val copy = source.task(id, anchor)!!.taskId
+        setRule(id, "FREQ=WEEKLY")
+
+        source.setCompletedInstance(id, anchor, completed = false)
+
+        assertThat(source.task(copy)!!.isClosed).isFalse()
+        assertThat(storedRule(id)).isEqualTo("FREQ=WEEKLY")
+    }
+
+    @Test
+    fun anOccurrenceThatIsGoneReadsAsNothing() {
+        val id = dailyTask()
+        val anchor = source.task(id)!!.occurrenceStart!!
+
+        assertThat(source.task(id, anchor + 30.hours)).isNull()
+    }
+
+    @Test
+    fun aSeriesStoredWithMillisecondsStillMovesOn() {
+        val id = legacySeries()
+        val task = source.task(id)!!
+
+        source.setCompletedInstance(id, task.occurrenceStart!!, completed = true)
+
+        assertSeriesMovedOn(id, task.occurrenceStart)
+    }
+
+    @Test
+    fun anUnknownOccurrenceDoesNotCloseTheSeries() {
+        val id = dailyTask()
+        val anchor = source.task(id)!!.occurrenceStart!!
+
+        assertThrows(TaskWriteFailedException::class.java) {
+            source.setCompletedInstance(id, anchor + 30.hours, completed = true)
+        }
+
+        assertThat(source.task(id)!!.isClosed).isFalse()
+    }
+
+    @Test
+    fun roundingALegacySeriesLeavesItUnmodified() {
+        val id = legacySeries()
+        val before = storedColumn(id, Tasks.LAST_MODIFIED)
+
+        source.task(id)
+
+        assertThat(storedStart(id)!! % 1000).isEqualTo(0L)
+        assertThat(storedColumn(id, Tasks.LAST_MODIFIED)).isEqualTo(before)
+    }
+
+    /** A daily series as builds before whole-second writes left it, already in sync with its server. */
+    private fun legacySeries(): Long {
+        val start = (Clock.System.now() + 1.hours).toEpochMilliseconds() / 1000 * 1000
+        return insertAsSyncAdapter(
+            Tasks.LIST_ID to listId,
+            Tasks.TITLE to "Old daily",
+            Tasks.DTSTART to start + 345,
+            Tasks.DUE to start + 3_600_345,
+            Tasks.TZ to "UTC",
+            Tasks.RRULE to "FREQ=DAILY",
+        ).also {
+            // The app's own reads may already have run the one-time rounding.
+            context.getSharedPreferences("provider_migrations", Context.MODE_PRIVATE).edit().clear().commit()
+        }
+    }
+
+    private fun insertAsSyncAdapter(vararg values: Pair<String, Any>): Long {
+        val uri = TasksContract.asSyncAdapter(
+            TasksContract.tasksUri(resolver.resolve()!!.authority),
+            TasksContract.LOCAL_ACCOUNT_NAME,
+            TasksContract.LOCAL_ACCOUNT_TYPE,
+        )
+        val cv = ContentValues().apply {
+            values.forEach { (k, v) -> if (v is Long) put(k, v) else put(k, v.toString()) }
+        }
+        return context.contentResolver.insert(uri, cv)!!.lastPathSegment!!.toLong()
+    }
+
+    /** Edits the series' first occurrence on its own and returns the override that makes. */
+    private fun overrideOf(id: Long): Task {
+        val task = source.task(id)!!
+        source.updateInstance(
+            id,
+            task.occurrenceStart!!,
+            TaskForm(title = "Just this once", listId = listId, start = task.start, due = task.due),
+            reminders = null,
+        )
+        return source.tasks(TaskQuery(listId = listId)).single { it.seriesId == id && it.taskId != id }
+    }
+
+    @Test
+    fun anOverrideBelongsToItsSeries() {
+        val id = dailyTask()
+        val anchor = source.task(id)!!.occurrenceStart
+
+        val override = overrideOf(id)
+
+        assertThat(override.title).isEqualTo("Just this once")
+        assertThat(override.isOccurrence).isTrue()
+        assertThat(override.occurrenceStart).isEqualTo(anchor)
+        assertThat(override.recurrenceRule).isEqualTo("FREQ=DAILY")
+        assertThat(source.task(override.taskId, override.occurrenceStart)?.taskId).isEqualTo(override.taskId)
+    }
+
+    @Test
+    fun anAllDayDueAddedToTheSeriesStaysOnItsDay() {
+        val day = Clock.System.now().toEpochMilliseconds() / 86_400_000 * 86_400_000
+        val id = source.insertTask(
+            TaskForm(title = "All day", listId = listId, start = Instant.fromEpochMilliseconds(day), isAllDay = true, rrule = "FREQ=DAILY"),
+            reminders = null,
+        )
+        val task = source.task(id)!!
+        // Local midnight the next day, as the date picker hands it over.
+        val localMidnight = Instant.fromEpochMilliseconds(day + 86_400_000 - java.util.TimeZone.getDefault().getOffset(day))
+
+        source.updateSeries(
+            id,
+            task.occurrenceStart!!,
+            TaskForm(title = "All day", listId = listId, start = task.start, due = localMidnight, isAllDay = true, rrule = "FREQ=DAILY"),
+            reminders = null,
+        )
+
+        assertThat(storedColumn(id, Tasks.DUE)!!.toLong() % 86_400_000).isEqualTo(0L)
+    }
+
+    @Test
+    fun savingAllFromAnOverrideKeepsTheSeriesWhereItIs() {
+        val id = dailyTask()
+        val before = storedStart(id)
+        val override = overrideOf(id)
+
+        source.updateSeries(
+            id,
+            override.occurrenceStart!!,
+            TaskForm(title = "Renamed", listId = listId, start = override.start, due = override.due, rrule = override.recurrenceRule),
+            reminders = null,
+        )
+
+        assertThat(storedStart(id)).isEqualTo(before)
+        assertThat(storedRule(id)).isEqualTo("FREQ=DAILY")
+    }
+
+    @Test
+    fun deletingAnOverrideExcludesItsOccurrence() {
+        val id = dailyTask()
+        val override = overrideOf(id)
+
+        source.deleteOverride(override.taskId, override.occurrenceStart!!)
+
+        // Excluded on the master, not just the override dropped.
+        assertThat(storedColumn(id, Tasks.EXDATE)).isNotNull()
+        val open = source.tasks(TaskQuery(listId = listId)).filter { it.seriesId == id }
+        assertThat(open.map { it.occurrenceStart }).doesNotContain(override.occurrenceStart)
+        assertThat(storedRule(id)).isEqualTo("FREQ=DAILY")
+    }
+
+    @Test
+    fun editingAnOverrideAgainKeepsItsNewTagsAndReminders() {
+        val id = dailyTask()
+        val override = overrideOf(id)
+        val form = TaskForm(
+            title = override.title,
+            listId = listId,
+            start = override.start,
+            due = override.due,
+            tags = listOf("urgent"),
+            reminders = listOf(TaskReminder(minutesBefore = 15)),
+        )
+
+        source.updateInstance(override.taskId, override.occurrenceStart!!, form, form.reminders)
+
+        assertThat(source.task(override.taskId)!!.tags).containsExactly("urgent")
+        assertThat(source.reminders()[override.taskId]).containsExactly(TaskReminder(minutesBefore = 15))
+        assertThat(source.task(id)?.tags.orEmpty()).isEmpty()
+    }
+
+    @Test
+    fun anOccurrenceIsFoundByItsAnchorToTheSecond() {
+        val id = dailyTask()
+        val override = overrideOf(id)
+
+        val found = source.task(id, override.occurrenceStart!! + 345.milliseconds)
+
+        assertThat(found?.taskId).isEqualTo(override.taskId)
+    }
+
+    private fun storedStart(taskId: Long): Long? = storedColumn(taskId, Tasks.DTSTART)?.toLong()
+
+    private fun storedColumn(taskId: Long, column: String): String? = context.contentResolver.query(
+        TasksContract.tasksUri(resolver.resolve()!!.authority),
+        arrayOf(column),
+        "${Tasks.ID} = ?",
+        arrayOf(taskId.toString()),
+        null,
+    )?.use { c -> if (c.moveToFirst()) c.getString(0) else null }
+
+    private fun assertSeriesMovedOn(id: Long, completed: Instant) {
+        assertThat(storedRule(id)).isNotNull()
+        val open = source.tasks(TaskQuery(listId = listId)).filter { it.seriesId == id }
+        assertThat(open).isNotEmpty()
+        assertThat(open.minOf { it.occurrenceStart!! }).isGreaterThan(completed)
+    }
+}

app/src/androidTest/java/de/jeanlucmakiola/agendula/data/tasks/provider/AgendulaTaskProviderTest.kt

@@ -124,6 +124,7 @@ class AgendulaTaskProviderTest {
 
     @Test
     fun aBadWriteReachesTheClientAsIllegalArgument() {
+        assumeTrue("Agendula is running on an external provider", ownStore())
         // No list has this id, so nothing in the app's real store is touched.
         val values = ContentValues().apply {
             put("list_id", Long.MAX_VALUE)
@@ -135,6 +136,7 @@ class AgendulaTaskProviderTest {
 
     @Test
     fun aFailingBatchWritesNothing() {
+        assumeTrue("Agendula is running on an external provider", ownStore())
         val missingList = Long.MAX_VALUE
         val operations = arrayListOf(
             ContentProviderOperation.newInsert(uri("tasks")).withValue("list_id", missingList).withValue("title", "a").build(),
@@ -145,6 +147,9 @@ class AgendulaTaskProviderTest {
             .use { assertThat(it.count).isEqualTo(0) }
     }
 
+    /** Whether the app's own store is active: only then does the provider take writes at all. */
+    private fun ownStore() = runCatching { resolver.delete(uri("tasks"), "_id = -1", null) }.isSuccess
+
     /** stdout and stderr of [command]; the denial lands on stderr. */
     private fun shell(command: String): String {
         val (stdout, stdin, stderr) = InstrumentationRegistry.getInstrumentation().uiAutomation

app/src/androidTest/java/de/jeanlucmakiola/agendula/data/tasks/provider/ProviderWriterTest.kt

@@ -734,9 +734,11 @@ class ProviderWriterTest {
         val series = db.tasks().insert(
             TaskEntity(listId = home, uid = "f", title = "Floating", due = today, rrule = "FREQ=DAILY;COUNT=3", timezone = null),
         )
-        val row = readRow(ProviderTable.INSTANCES, instanceId(today))
+        // The second occurrence: ticking the first moves the series on instead.
+        val second = today + 1.days
+        val row = readRow(ProviderTable.INSTANCES, instanceId(second))
 
-        writer.update(ProviderTable.INSTANCES, row + (Tasks.STATUS to 2), byId(ProviderTable.INSTANCES, instanceId(today)), null)
+        writer.update(ProviderTable.INSTANCES, row + (Tasks.STATUS to 2), byId(ProviderTable.INSTANCES, instanceId(second)), null)
         writer.update(ProviderTable.TASKS, mapOf(Tasks.TITLE to "Still floating"), byId(ProviderTable.TASKS, series), null)
 
         assertThat(db.tasks().overrides(series).single().status).isEqualTo(TaskStatus.COMPLETED)
@@ -795,6 +797,20 @@ class ProviderWriterTest {
 
     private fun store() = RoomTasksDataSource(db)
 
+    @Test
+    fun reopeningATickedOccurrencesCopyMovesTheSeriesBack() {
+        val series = db.tasks().insert(
+            TaskEntity(listId = home, uid = "w", title = "Weekly", dtstart = today, rrule = "FREQ=WEEKLY", timezone = "UTC"),
+        )
+        store().setCompletedInstance(series, today, true)
+        val copy = db.tasks().allIn(home).single { it.id != series }
+
+        writer.update(ProviderTable.TASKS, mapOf(Tasks.STATUS to 0), byId(ProviderTable.TASKS, copy.id), null)
+
+        assertThat(task(series)!!.dtstart).isEqualTo(today)
+        assertThat(task(copy.id)).isNull()
+    }
+
     /** Every column of one published row, as a client that read it would write it back. */
     private fun readRow(table: ProviderTable, id: Long): Map<String, Any?> {
         val columns = table.columns.map { it.first }

app/src/androidTest/java/de/jeanlucmakiola/agendula/data/tasks/room/RoomTasksDataSourceTest.kt

@@ -211,7 +211,8 @@ class RoomTasksDataSourceTest {
     @Test
     fun completingOneOccurrenceLeavesTheRestOfTheSeriesOpen() {
         val id = source.insertTask(form(title = "Water the plants"))
-        makeRecurring(id, now)
+        // Overdue occurrences before it, so it is not the first and forks an override.
+        makeRecurring(id, now - 14.days)
         val open = { source.tasks(TaskQuery(listId = listId)).filter { it.taskId == id } }
         val before = open()
         val target = before.first { it.distanceFromCurrent == 0 }
@@ -237,7 +238,8 @@ class RoomTasksDataSourceTest {
     @Test
     fun reopeningACompletedOccurrenceReusesItsOverride() {
         val id = source.insertTask(form(title = "Water the plants"))
-        makeRecurring(id, now)
+        // Overdue occurrences before it, so it is not the first and forks an override.
+        makeRecurring(id, now - 14.days)
         val target = source.tasks(TaskQuery(listId = listId))
             .first { it.taskId == id && it.distanceFromCurrent == 0 }
 
@@ -254,7 +256,8 @@ class RoomTasksDataSourceTest {
     @Test
     fun cancellingOneOccurrenceLeavesTheRestOfTheSeriesOpen() {
         val id = source.insertTask(form(title = "Water the plants"))
-        makeRecurring(id, now)
+        // Overdue occurrences before it, so it is not the first and forks an override.
+        makeRecurring(id, now - 14.days)
         val target = source.tasks(TaskQuery(listId = listId))
             .first { it.taskId == id && it.distanceFromCurrent == 0 }
 
@@ -269,6 +272,621 @@ class RoomTasksDataSourceTest {
         assertThat(db.tasks().override(id, target.occurrenceStart)!!.status).isEqualTo(TaskStatus.NEEDS_ACTION)
     }
 
+    @Test
+    fun completingTheFirstOccurrenceMovesTheSeriesOnAndKeepsACopy() {
+        val at = now
+        val id = source.insertTask(form(title = "Water the plants"))
+        makeRecurring(id, at, rule = "FREQ=WEEKLY;COUNT=4")
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        val master = db.tasks().entity(id)!!
+        assertThat(master.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+        assertThat(master.dtstart).isEqualTo(at + 7.days)
+        assertThat(master.due).isEqualTo(at + 8.days)
+        assertThat(master.rrule).isEqualTo("FREQ=WEEKLY;COUNT=3")
+        assertThat(master.isDirty).isTrue()
+        assertThat(db.tasks().overrides(id)).isEmpty()
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+        assertThat(copy.status).isEqualTo(TaskStatus.COMPLETED)
+        assertThat(copy.uid).isNotEqualTo(master.uid)
+        assertThat(copy.rrule).isNull()
+        assertThat(copy.masterId).isNull()
+        assertThat(copy.dtstart).isEqualTo(at)
+        assertThat(copy.title).isEqualTo("Water the plants")
+    }
+
+    @Test
+    fun reopeningTheOccurrenceMovesTheSeriesBack() {
+        val at = now
+        val id = source.insertTask(form(title = "Water the plants"))
+        makeRecurring(id, at, rule = "FREQ=WEEKLY;COUNT=4")
+        source.setCompletedInstance(id, at, completed = true)
+
+        source.setCompletedInstance(id, at, completed = false)
+
+        val master = db.tasks().entity(id)!!
+        assertThat(master.dtstart).isEqualTo(at)
+        assertThat(master.rrule).isEqualTo("FREQ=WEEKLY;COUNT=4")
+        assertThat(db.tasks().allIn(listId).filter { !it.isDeleted }.map { it.id }).containsExactly(id)
+    }
+
+    @Test
+    fun completingAnEditedFirstOccurrenceKeepsItsEdits() {
+        val at = now
+        val id = source.insertTask(form(title = "Water the plants"))
+        makeRecurring(id, at)
+        source.updateInstance(id, at, TaskForm(title = "Water the cactus", listId = listId, start = at, due = at + 1.days), reminders = null)
+        val override = db.tasks().override(id, at)!!
+
+        source.setCompletedInstance(override.id, at, completed = true)
+
+        assertThat(db.tasks().overrides(id)).isEmpty()
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 7.days)
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+        assertThat(copy.title).isEqualTo("Water the cactus")
+        assertThat(copy.status).isEqualTo(TaskStatus.COMPLETED)
+    }
+
+    @Test
+    fun completingTheLastOccurrenceClosesTheSeries() {
+        val at = now
+        val id = source.insertTask(form(title = "Water the plants"))
+        makeRecurring(id, at, rule = "FREQ=WEEKLY;COUNT=1")
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        assertThat(db.tasks().entity(id)!!.status).isEqualTo(TaskStatus.COMPLETED)
+        assertThat(db.tasks().allIn(listId).map { it.id }).containsExactly(id)
+
+        source.setCompletedInstance(id, at, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+    }
+
+    @Test
+    fun cancellingTheFirstOccurrenceMovesTheSeriesOn() {
+        val at = now
+        val id = source.insertTask(form(title = "Water the plants"))
+        makeRecurring(id, at)
+
+        source.setCancelledInstance(id, at, cancelled = true)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 7.days)
+        assertThat(db.tasks().allIn(listId).single { it.id != id }.status).isEqualTo(TaskStatus.CANCELLED)
+    }
+
+    @Test
+    fun aSeriesWhoseStartWasDeletedMovesPastTheTickedOne() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.deleteOccurrence(id, at)
+
+        source.setCompletedInstance(id, at + 7.days, completed = true)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 14.days)
+    }
+
+    @Test
+    fun countSpendsExcludedOccurrencesAndReopeningRestoresThem() {
+        val at = now
+        val id = source.insertTask(form(title = "Daily"))
+        makeRecurring(id, at, rule = "FREQ=DAILY;COUNT=4")
+        source.deleteOccurrence(id, at + 1.days)
+        val before = db.tasks().entity(id)!!
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        val moved = db.tasks().entity(id)!!
+        assertThat(moved.dtstart).isEqualTo(at + 2.days)
+        assertThat(moved.rrule).isEqualTo("FREQ=DAILY;COUNT=2")
+
+        source.setCompletedInstance(id, at, completed = false)
+
+        val back = db.tasks().entity(id)!!
+        assertThat(back.dtstart).isEqualTo(before.dtstart)
+        assertThat(back.rrule).isEqualTo(before.rrule)
+        assertThat(back.exdate).isEqualTo(before.exdate)
+    }
+
+    @Test
+    fun reopeningTheCopyItselfMovesTheSeriesBack() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+
+        source.setCompleted(copy.id, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+        assertThat(db.tasks().allIn(listId).filter { !it.isDeleted }.map { it.id }).containsExactly(id)
+    }
+
+    @Test
+    fun undoingAnEditedOccurrenceBringsItsOverrideBack() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.updateInstance(id, at, TaskForm(title = "Just this once", listId = listId, start = at, due = at + 1.days), reminders = null)
+        val override = db.tasks().override(id, at)!!
+        source.setCompletedInstance(override.id, at, completed = true)
+
+        source.setCompletedInstance(override.id, at, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+        val back = db.tasks().override(id, at)!!
+        assertThat(back.title).isEqualTo("Just this once")
+        assertThat(back.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+    }
+
+    @Test
+    fun aStaleTickOfAnOccurrenceMovedPastForksNothing() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        assertThat(db.tasks().overrides(id)).isEmpty()
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 7.days)
+    }
+
+    @Test
+    fun occurrencesClosedBeforeTheUpdateNoLongerHoldTheSeriesBack() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        val master = db.tasks().entity(id)!!
+        db.tasks().insert(
+            master.copy(id = 0, masterId = id, recurrenceId = at, rrule = null, status = TaskStatus.COMPLETED, completedAt = at),
+        )
+
+        source.setCompletedInstance(id, at + 7.days, completed = true)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 14.days)
+        assertThat(db.tasks().overrides(id)).isEmpty()
+        assertThat(db.tasks().allIn(listId).count { it.id != id && it.status == TaskStatus.COMPLETED }).isEqualTo(2)
+    }
+
+    @Test
+    fun theTickedOccurrenceStillResolvesToItsCopy() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        val occurrence = source.task(id, at)!!.occurrenceStart!!
+
+        source.setCompletedInstance(id, occurrence, completed = true)
+
+        val found = source.task(id, occurrence)!!
+        assertThat(found.taskId).isNotEqualTo(id)
+        assertThat(found.isCompleted).isTrue()
+        assertThat(found.occurrenceStart).isEqualTo(occurrence)
+    }
+
+    @Test
+    fun theNextOccurrenceStartsWithoutTheLastOnesProgress() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        db.tasks().update(db.tasks().entity(id)!!.copy(status = TaskStatus.IN_PROCESS, percentComplete = 60))
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        val master = db.tasks().entity(id)!!
+        assertThat(master.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+        assertThat(master.percentComplete).isNull()
+    }
+
+    @Test
+    fun theCopyOfAnEditedOccurrenceKeepsItsOwnProperties() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        val master = db.tasks().entity(id)!!
+        val override = db.tasks().insert(
+            master.copy(id = 0, masterId = id, recurrenceId = at, rrule = null, unknownProperties = "COMMENT:only this one"),
+        )
+
+        source.setCompletedInstance(override, at, completed = true)
+
+        assertThat(db.tasks().entity(override)!!.unknownProperties!!.lines()).contains("COMMENT:only this one")
+    }
+
+    @Test
+    fun aRuleWithAGapOfMoreThanTenYearsKeepsGoing() {
+        val at = now
+        val id = source.insertTask(form(title = "Rare"))
+        makeRecurring(id, at, rule = "FREQ=YEARLY;INTERVAL=11")
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        val master = db.tasks().entity(id)!!
+        assertThat(master.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+        assertThat(master.dtstart!!).isGreaterThan(at + (366 * 10).days)
+    }
+
+    @Test
+    fun reopeningKeepsWhatChangedOnTheSeriesSince() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        source.deleteOccurrence(id, at + 28.days)
+
+        source.setCompletedInstance(id, at, completed = false)
+
+        val master = db.tasks().entity(id)!!
+        assertThat(master.dtstart).isEqualTo(at)
+        assertThat(master.exdate).isNotNull()
+        assertThat(source.tasks(TaskQuery(listId = listId)).map { it.occurrenceStart?.epochSeconds })
+            .doesNotContain((at + 28.days).epochSeconds)
+    }
+
+    @Test
+    fun aSeriesWithExtraDatesKeepsTheOverrideModel() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        db.tasks().update(db.tasks().entity(id)!!.copy(rdate = "20991231T090000Z"))
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+        assertThat(db.tasks().overrides(id).single().status).isEqualTo(TaskStatus.COMPLETED)
+    }
+
+    @Test
+    fun aDeletedSeriesIsNotBroughtBackByItsCopy() {
+        val at = now
+        val work = syncedList()
+        val id = source.insertTask(form(title = "Weekly").copy(listId = work))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        val copy = db.tasks().allIn(work).single { it.id != id }
+        source.deleteWithSubtasks(id)
+
+        source.setCompleted(copy.id, completed = false)
+
+        assertThat(db.tasks().entity(copy.id)!!.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+        assertThat(db.tasks().entity(copy.id)!!.isDeleted).isFalse()
+    }
+
+    @Test
+    fun aSeriesEndedOnItsLastOpenOccurrenceReopens() {
+        val at = now
+        val id = source.insertTask(form(title = "Three"))
+        makeRecurring(id, at, rule = "FREQ=WEEKLY;COUNT=3")
+        val master = db.tasks().entity(id)!!
+        listOf(at, at + 7.days).forEach { done ->
+            db.tasks().insert(master.copy(id = 0, masterId = id, recurrenceId = done, rrule = null, status = TaskStatus.COMPLETED))
+        }
+
+        source.setCompletedInstance(id, at + 14.days, completed = true)
+        assertThat(db.tasks().entity(id)!!.status).isEqualTo(TaskStatus.COMPLETED)
+        // Nothing to move past: the closed occurrences stay the series' own.
+        assertThat(db.tasks().overrides(id)).hasSize(2)
+
+        source.setCompletedInstance(id, at + 14.days, completed = false)
+        assertThat(db.tasks().entity(id)!!.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+    }
+
+    @Test
+    fun reopeningAnOlderCopyReopensItOnItsOwn() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        source.setCompletedInstance(id, at + 7.days, completed = true)
+
+        source.setCompletedInstance(id, at, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 14.days)
+        val reopened = db.tasks().allIn(listId).single { it.id != id && it.dtstart == at }
+        assertThat(reopened.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+        assertThat(db.advancedOccurrences().forCopy(reopened.id)!!.undoable).isFalse()
+    }
+
+    @Test
+    fun aSeriesMovesPastOccurrencesAlreadyClosedAfterIt() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        val master = db.tasks().entity(id)!!
+        db.tasks().insert(master.copy(id = 0, masterId = id, recurrenceId = at + 7.days, rrule = null, status = TaskStatus.COMPLETED))
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 14.days)
+        assertThat(db.tasks().overrides(id)).isEmpty()
+    }
+
+    @Test
+    fun aDetachedOccurrenceDeletedSinceStaysExcluded() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        val master = db.tasks().entity(id)!!
+        val earlier = db.tasks().insert(master.copy(id = 0, masterId = id, recurrenceId = at, rrule = null, status = TaskStatus.COMPLETED))
+        source.setCompletedInstance(id, at + 7.days, completed = true)
+        source.deleteTask(earlier)
+
+        source.setCompletedInstance(id, at + 7.days, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+        assertThat(source.tasks(TaskQuery(listId = listId)).map { it.occurrenceStart?.epochSeconds })
+            .doesNotContain(at.epochSeconds)
+    }
+
+    @Test
+    fun aNestedComponentKeepsItsOwnUid() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        val master = db.tasks().entity(id)!!
+        val override = db.tasks().insert(
+            master.copy(
+                id = 0, masterId = id, recurrenceId = at, rrule = null,
+                unknownProperties = "BEGIN:VALARM\r\nUID:alarm-1\r\nACTION:AUDIO\r\nEND:VALARM",
+            ),
+        )
+
+        source.setCompletedInstance(override, at, completed = true)
+
+        assertThat(db.tasks().entity(override)!!.unknownProperties!!.lines()).contains("UID:alarm-1")
+    }
+
+    @Test
+    fun reopeningTheCopyInTheEditorMovesTheSeriesBack() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+
+        source.updateTask(
+            copy.id,
+            TaskForm(title = copy.title.orEmpty(), listId = listId, start = copy.dtstart, due = copy.due, percentComplete = 0),
+            reminders = null,
+        )
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+    }
+
+    @Test
+    fun anEditedCopyReopensAsATaskOfItsOwn() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+        db.tasks().update(copy.copy(description = "notes"))
+        source.updateTask(copy.id, TaskForm(title = "Renamed", listId = listId, start = copy.dtstart, due = copy.due, percentComplete = 100), null)
+
+        source.setCompleted(copy.id, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 7.days)
+        assertThat(db.tasks().entity(copy.id)!!.title).isEqualTo("Renamed")
+        assertThat(db.tasks().entity(copy.id)!!.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+    }
+
+    @Test
+    fun aCopyWithASubtaskReopensOnItsOwn() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+        source.insertTask(form(title = "Part").copy(parentId = copy.id))
+
+        source.setCompleted(copy.id, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 7.days)
+        assertThat(db.tasks().entity(copy.id)!!.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+    }
+
+    @Test
+    fun aRecurringSubtaskKeepsTheOverrideModel() {
+        val at = now
+        val parent = source.insertTask(form(title = "Parent"))
+        val id = source.insertTask(form(title = "Daily part").copy(parentId = parent))
+        makeRecurring(id, at, rule = "FREQ=DAILY")
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+        assertThat(db.tasks().overrides(id)).hasSize(1)
+    }
+
+    @Test
+    fun aSeriesReshapedSinceIsNotMovedBack() {
+        val at = now
+        val id = source.insertTask(form(title = "Ten"))
+        makeRecurring(id, at, rule = "FREQ=WEEKLY;COUNT=10")
+        source.setCompletedInstance(id, at, completed = true)
+        db.tasks().update(db.tasks().entity(id)!!.copy(rrule = "FREQ=DAILY;COUNT=5"))
+
+        source.setCompletedInstance(id, at, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 7.days)
+        assertThat(db.tasks().entity(id)!!.rrule).isEqualTo("FREQ=DAILY;COUNT=5")
+        assertThat(db.tasks().allIn(listId).single { it.id != id }.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+    }
+
+    @Test
+    fun tickingTheSameOccurrenceAgainKeepsOneRecord() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        source.setCompletedInstance(id, at, completed = false)
+
+        source.setCompletedInstance(id, at, completed = true)
+
+        assertThat(db.advancedOccurrences().forMaster(id)).hasSize(1)
+        source.setCompletedInstance(id, at, completed = false)
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+    }
+
+    @Test
+    fun anEditedCopyStillStandsForItsOccurrence() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+
+        db.tasks().update(copy.copy(title = "Watered"))
+
+        assertThat(source.task(id, at)!!.taskId).isEqualTo(copy.id)
+    }
+
+    @Test
+    fun aDetachedOccurrenceWithALifeOfItsOwnStaysOut() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        val master = db.tasks().entity(id)!!
+        val earlier = db.tasks().insert(master.copy(id = 0, masterId = id, recurrenceId = at, rrule = null, status = TaskStatus.COMPLETED))
+        source.setCompletedInstance(id, at + 7.days, completed = true)
+        source.insertTask(form(title = "Follow-up").copy(parentId = earlier))
+
+        source.setCompletedInstance(id, at + 7.days, completed = false)
+
+        assertThat(db.tasks().entity(earlier)!!.masterId).isNull()
+        assertThat(source.tasks(TaskQuery(listId = listId)).map { it.occurrenceStart?.epochSeconds })
+            .doesNotContain(at.epochSeconds)
+    }
+
+    @Test
+    fun reopeningAnotherOverrideOfAFinishedSeriesLeavesItsLastOccurrenceClosed() {
+        val at = now
+        val id = source.insertTask(form(title = "Three"))
+        makeRecurring(id, at, rule = "FREQ=WEEKLY;COUNT=3")
+        val master = db.tasks().entity(id)!!
+        val last = db.tasks().insert(master.copy(id = 0, masterId = id, recurrenceId = at + 14.days, rrule = null, status = TaskStatus.COMPLETED))
+        source.setCompletedInstance(id, at, completed = true)
+        source.setCompletedInstance(id, at + 7.days, completed = true)
+        assertThat(db.tasks().entity(id)!!.status).isEqualTo(TaskStatus.COMPLETED)
+
+        source.setCompletedInstance(last, at + 14.days, completed = false)
+
+        assertThat(db.tasks().entity(last)!!.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+        assertThat(db.tasks().entity(id)!!.status).isEqualTo(TaskStatus.COMPLETED)
+        assertThat(source.task(id, at + 7.days)!!.isClosed).isTrue()
+    }
+
+    @Test
+    fun reopeningTheCopyWithNewRemindersKeepsThem() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+
+        source.updateTask(
+            copy.id,
+            TaskForm(title = copy.title.orEmpty(), listId = listId, start = copy.dtstart, due = copy.due, percentComplete = 0),
+            reminders = listOf(TaskReminder(minutesBefore = 30)),
+        )
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at + 7.days)
+        assertThat(db.tasks().entity(copy.id)!!.status).isEqualTo(TaskStatus.NEEDS_ACTION)
+        assertThat(db.alarms().forTask(copy.id)).hasSize(1)
+    }
+
+    @Test
+    fun aDetachedOccurrenceReopenedSinceStaysOut() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        val master = db.tasks().entity(id)!!
+        val earlier = db.tasks().insert(master.copy(id = 0, masterId = id, recurrenceId = at, rrule = null, status = TaskStatus.COMPLETED))
+        source.setCompletedInstance(id, at + 7.days, completed = true)
+        source.setCompleted(earlier, completed = false)
+
+        source.setCompletedInstance(id, at + 7.days, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+        assertThat(db.tasks().entity(earlier)!!.masterId).isNull()
+        assertThat(source.tasks(TaskQuery(listId = listId)).filter { it.taskId == id }.map { it.occurrenceStart?.epochSeconds })
+            .doesNotContain(at.epochSeconds)
+    }
+
+    @Test
+    fun anOccurrenceWhoseCopyIsGoneReadsAsNothing() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        source.deleteTask(db.tasks().allIn(listId).single { it.id != id }.id)
+
+        assertThat(source.task(id, at)).isNull()
+    }
+
+    @Test
+    fun catchingUpOnAnOverdueSeriesKeepsEachTickUndoable() {
+        val at = now - 56.days
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        val first = db.tasks().allIn(listId).single { it.id != id }
+        source.setCompletedInstance(id, at + 7.days, completed = true)
+
+        assertThat(db.advancedOccurrences().forCopy(first.id)).isNotNull()
+    }
+
+    @Test
+    fun aRuleTheServerReorderedStillMovesBack() {
+        val at = now
+        val id = source.insertTask(form(title = "Ten"))
+        makeRecurring(id, at, rule = "FREQ=WEEKLY;COUNT=10")
+        source.setCompletedInstance(id, at, completed = true)
+        db.tasks().update(db.tasks().entity(id)!!.copy(rrule = "count=9;FREQ=WEEKLY"))
+
+        source.setCompletedInstance(id, at, completed = false)
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+    }
+
+    @Test
+    fun reopeningWithProgressKeepsIt() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        source.setCompletedInstance(id, at, completed = true)
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+
+        source.updateTask(
+            copy.id,
+            TaskForm(title = copy.title.orEmpty(), listId = listId, start = copy.dtstart, due = copy.due, percentComplete = 40),
+            reminders = null,
+        )
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+        assertThat(db.tasks().override(id, at)!!.percentComplete).isEqualTo(40)
+    }
+
+    @Test
+    fun theEditorsTidyingDoesNotCountAsAnEdit() {
+        val at = now
+        val id = source.insertTask(form(title = "Weekly"))
+        makeRecurring(id, at)
+        db.tasks().update(db.tasks().entity(id)!!.copy(description = "From the server\n"))
+        source.setCompletedInstance(id, at, completed = true)
+        val copy = db.tasks().allIn(listId).single { it.id != id }
+
+        source.updateTask(
+            copy.id,
+            TaskForm(title = copy.title.orEmpty(), listId = listId, description = copy.description, start = copy.dtstart, due = copy.due, percentComplete = 0),
+            reminders = null,
+        )
+
+        assertThat(db.tasks().entity(id)!!.dtstart).isEqualTo(at)
+    }
+
     @Test
     fun completingANonRecurringTaskThroughTheInstancePathWritesTheRowItself() {
         val id = source.insertTask(form(title = "Buy milk", due = now + 1.days))
@@ -657,10 +1275,12 @@ class RoomTasksDataSourceTest {
         val outOfWindow = db.tasks().insert(
             TaskEntity(listId = listId, uid = "old", title = "old", dtstart = longAgo, rrule = "FREQ=DAILY;COUNT=2", parentId = parent),
         )
+        val at = now
         val series = db.tasks().insert(
-            TaskEntity(listId = listId, uid = "s", title = "s", dtstart = now, rrule = "FREQ=DAILY;COUNT=3", parentId = parent),
+            TaskEntity(listId = listId, uid = "s", title = "s", dtstart = at, rrule = "FREQ=DAILY;COUNT=3", parentId = parent),
         )
-        source.setCompletedInstance(series, now, true)
+        // Not the first occurrence, so it forks an override rather than a copy.
+        source.setCompletedInstance(series, at + 1.days, true)
         val plain = source.insertTask(form("plain").copy(parentId = parent))
 
         assertThat(source.subtaskIds(parent)).containsExactly(outOfWindow, series, plain)

app/src/androidTest/java/de/jeanlucmakiola/agendula/data/tasks/room/TasksDatabaseMigrationTest.kt

@@ -162,6 +162,18 @@ class TasksDatabaseMigrationTest {
         }
     }
 
+    @Test
+    fun addsTheAdvancedOccurrencesTable() {
+        helper.createDatabase(TEST_DB, 4).close()
+
+        helper.runMigrationsAndValidate(TEST_DB, 5, true, TasksDatabase.MIGRATION_4_5).use { db ->
+            db.query("SELECT COUNT(*) FROM advanced_occurrences").use { c ->
+                assertThat(c.moveToFirst()).isTrue()
+                assertThat(c.getInt(0)).isEqualTo(0)
+            }
+        }
+    }
+
     private companion object {
         const val TEST_DB = "migration-test.db"
     }

app/src/androidTestFull/java/de/jeanlucmakiola/agendula/data/sync/RoomSyncStoreAdvancesTest.kt

@@ -0,0 +1,90 @@
+package de.jeanlucmakiola.agendula.data.sync
+
+import android.content.Context
+import androidx.room.Room
+import androidx.test.core.app.ApplicationProvider
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.google.common.truth.Truth.assertThat
+import de.jeanlucmakiola.agendula.data.tasks.room.RoomTasksDataSource
+import de.jeanlucmakiola.agendula.data.tasks.room.TaskEntity
+import de.jeanlucmakiola.agendula.data.tasks.room.TaskListEntity
+import de.jeanlucmakiola.agendula.data.tasks.room.TasksDatabase
+import org.junit.After
+import org.junit.Test
+import org.junit.runner.RunWith
+import kotlin.time.Clock
+import kotlin.time.Duration.Companion.seconds
+import kotlin.time.Instant
+
+/** A server version replacing a moved series takes the moves' copies with it (#137). */
+@RunWith(AndroidJUnit4::class)
+class RoomSyncStoreAdvancesTest {
+
+    private val db = Room.inMemoryDatabaseBuilder(ApplicationProvider.getApplicationContext<Context>(), TasksDatabase::class.java)
+        .allowMainThreadQueries().build()
+
+    @After
+    fun tearDown() = db.close()
+
+    @Test
+    fun droppingAdvancesDeletesTheirCopies() {
+        val list = db.taskLists().insert(TaskListEntity(name = "Home", color = 0))
+        val at = Instant.fromEpochSeconds(Clock.System.now().epochSeconds)
+        val series = db.tasks().insert(TaskEntity(listId = list, uid = "s", title = "Weekly", dtstart = at, rrule = "FREQ=WEEKLY"))
+        RoomTasksDataSource(db).setCompletedInstance(series, at, true)
+        val copy = db.tasks().allIn(list).single { it.id != series }
+        db.tasks().update(copy.copy(href = "copy.ics"))
+
+        RoomSyncStore(db).dropAdvances(series)
+
+        assertThat(db.advancedOccurrences().forMaster(series)).isEmpty()
+        assertThat(db.tasks().entity(copy.id)!!.isDeleted).isTrue()
+    }
+
+    @Test
+    fun movesTheServerAlreadyHasStay() {
+        val list = db.taskLists().insert(TaskListEntity(name = "Home", color = 0))
+        val at = Instant.fromEpochSeconds(Clock.System.now().epochSeconds)
+        val series = db.tasks().insert(TaskEntity(listId = list, uid = "s", title = "Weekly", dtstart = at, rrule = "FREQ=WEEKLY"))
+        RoomTasksDataSource(db).setCompletedInstance(series, at, true)
+        val copy = db.tasks().allIn(list).single { it.id != series }
+        val store = RoomSyncStore(db)
+        store.markSynced(listOf(db.tasks().entity(series)!!), "s.ics", "1")
+
+        store.dropAdvances(series)
+
+        assertThat(db.tasks().entity(copy.id)!!.isDeleted).isFalse()
+    }
+
+    @Test
+    fun aCopyGivenASubtaskSurvivesTheServerWinning() {
+        val list = db.taskLists().insert(TaskListEntity(name = "Home", color = 0))
+        val at = Instant.fromEpochSeconds(Clock.System.now().epochSeconds)
+        val series = db.tasks().insert(TaskEntity(listId = list, uid = "s", title = "Weekly", dtstart = at, rrule = "FREQ=WEEKLY"))
+        RoomTasksDataSource(db).setCompletedInstance(series, at, true)
+        val copy = db.tasks().allIn(list).single { it.id != series }
+        db.tasks().insert(TaskEntity(listId = list, uid = "p", title = "Part", parentId = copy.id))
+
+        RoomSyncStore(db).dropAdvances(series)
+
+        assertThat(db.tasks().entity(copy.id)).isNotNull()
+        assertThat(db.tasks().entity(copy.id)!!.isDeleted).isFalse()
+    }
+
+    @Test
+    fun aTickDuringTheUploadIsNotSettled() {
+        val list = db.taskLists().insert(TaskListEntity(name = "Home", color = 0))
+        val at = Instant.fromEpochSeconds(Clock.System.now().epochSeconds)
+        val series = db.tasks().insert(
+            TaskEntity(listId = list, uid = "s", title = "Weekly", dtstart = at, rrule = "FREQ=WEEKLY", lastModified = at - 1.seconds),
+        )
+        val uploaded = db.tasks().entity(series)!!
+        RoomTasksDataSource(db).setCompletedInstance(series, at, true)
+        val store = RoomSyncStore(db)
+
+        store.markSynced(listOf(uploaded), "s.ics", "1")
+
+        assertThat(db.tasks().entity(series)!!.isDirty).isTrue()
+        assertThat(db.advancedOccurrences().forMaster(series).single().settled).isFalse()
+    }
+}

app/src/full/java/de/jeanlucmakiola/agendula/data/sync/CollectionSyncer.kt

@@ -609,7 +609,7 @@ class CollectionSyncer(
             // via `master_id`.
             val (buried, kept) = local.rows.partition { it.isDeleted && it.recurrenceId != null }
             if (buried.isNotEmpty()) store.deleteAll(buried.map { it.id })
-            store.markSynced(kept.map { it.id }, key, eTag)
+            store.markSynced(kept, key, eTag)
             // Both, because a resource that failed as a create was counted under
             // its UID and is now counted under its href. Clearing one would leak
             // the other into the store forever.
@@ -825,6 +825,7 @@ class CollectionSyncer(
             }
 
             pendingDiscard.remove(href)?.let { cause ->
+                store.dropAdvances(masterId)
                 report = report.copy(
                     discardedEdits = report.discardedEdits +
                         DiscardedEdit(uid, losing?.title, cause),

app/src/full/java/de/jeanlucmakiola/agendula/data/sync/SyncStore.kt

@@ -3,6 +3,9 @@ package de.jeanlucmakiola.agendula.data.sync
 import de.jeanlucmakiola.agendula.data.tasks.room.TaskAlarmEntity
 import de.jeanlucmakiola.agendula.data.tasks.room.TaskEntity
 import de.jeanlucmakiola.agendula.data.tasks.room.TasksDatabase
+import de.jeanlucmakiola.agendula.data.tasks.room.recurrenceAnchor
+import de.jeanlucmakiola.agendula.data.tasks.wholeSecond
+import kotlin.time.Duration.Companion.seconds
 import javax.inject.Inject
 
 /**
@@ -42,12 +45,21 @@ interface SyncStore {
     fun deleteAll(taskIds: List<Long>)
 
     /**
-     * Records href and ETag on a resource's rows, clearing `is_dirty`.
+     * Forgets how ticking occurrences moved [masterId]'s series, and deletes the
+     * copies that left: the server's version replaced the moved series, and its
+     * own overrides stand for those occurrences again. A row given a life of its
+     * own since stays.
+     */
+    fun dropAdvances(masterId: Long) = Unit
+
+    /**
+     * Records href and ETag on a resource's [rows], as they were serialised,
+     * clearing `is_dirty` on those that did not change since.
      *
      * The caller excludes any row it left out of the body — that row is deleted,
      * not marked synced.
      */
-    fun markSynced(taskIds: List<Long>, href: String?, eTag: String?)
+    fun markSynced(rows: List<TaskEntity>, href: String?, eTag: String?)
 
     fun setParent(taskId: Long, parentId: Long?)
 
@@ -97,8 +109,42 @@ class RoomSyncStore @Inject constructor(
         if (taskIds.isNotEmpty()) database.tasks().deleteAll(taskIds)
     }
 
-    override fun markSynced(taskIds: List<Long>, href: String?, eTag: String?) {
-        if (taskIds.isNotEmpty()) database.tasks().markSynced(taskIds, href, eTag)
+    override fun dropAdvances(masterId: Long) = database.runInTransaction {
+        val advanced = database.advancedOccurrences()
+        val tasks = database.tasks()
+        val master = tasks.entity(masterId)
+        // Only moves the server never saw, on copies that are still just that.
+        advanced.forMaster(masterId).filter { !it.settled && it.undoable }.forEach { record ->
+            // An edited detached row was unlinked already, so only untouched ones are named.
+            val rows = advanced.detachedFor(record.copyId).mapNotNull { it.taskId } + record.copyId
+            advanced.delete(record.copyId)
+            rows.mapNotNull { tasks.entity(it) }
+                .filter { row ->
+                    !row.isDeleted && tasks.subtasks(row.id).isEmpty() &&
+                        (master == null || row.listId == master.listId && row.parentId == master.parentId)
+                }
+                .forEach { row ->
+                    // Never uploaded: nobody to tell. Uploaded: a tombstone the next run deletes.
+                    if (row.href == null) tasks.delete(row.id)
+                    else tasks.update(row.copy(isDeleted = true, isDirty = true))
+                }
+        }
+    }
+
+    override fun markSynced(rows: List<TaskEntity>, href: String?, eTag: String?) {
+        if (rows.isEmpty()) return
+        database.runInTransaction {
+            database.tasks().markSynced(rows.map { it.id }, href, eTag)
+            // Changed while the body was on its way, by a tick or an edit: still owed to the server.
+            rows.forEach { database.tasks().dirtyIfChangedSince(it.id, it.lastModified) }
+            // The server now holds each series as far as the body moved it; a tick
+            // since is still the device's own.
+            rows.filter { it.recurrenceId == null }.forEach { row ->
+                val anchor = row.recurrenceAnchor
+                if (anchor == null) database.advancedOccurrences().settle(listOf(row.id))
+                else database.advancedOccurrences().settleUpTo(row.id, wholeSecond(anchor) + 1.seconds)
+            }
+        }
     }
 
     override fun setParent(taskId: Long, parentId: Long?) {

app/src/main/java/de/jeanlucmakiola/agendula/data/di/DataModule.kt

@@ -89,7 +89,7 @@ object DataProvideModule {
             // backed-up `.db` does not. The backup rules carry all three files and
             // the app checkpoints on ON_STOP.
             .setJournalMode(RoomDatabase.JournalMode.WRITE_AHEAD_LOGGING)
-            .addMigrations(TasksDatabase.MIGRATION_1_2, TasksDatabase.MIGRATION_2_3, TasksDatabase.MIGRATION_3_4)
+            .addMigrations(TasksDatabase.MIGRATION_1_2, TasksDatabase.MIGRATION_2_3, TasksDatabase.MIGRATION_3_4, TasksDatabase.MIGRATION_4_5)
             .build()
 
     /**

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/AndroidTasksDataSource.kt

@@ -7,9 +7,12 @@ import android.content.ContentUris
 import android.content.ContentValues
 import android.content.Context
 import android.database.ContentObserver
+import android.database.Cursor
+import android.database.sqlite.SQLiteException
 import android.net.Uri
 import android.os.Handler
 import android.os.Looper
+import android.util.Log
 import dagger.hilt.android.qualifiers.ApplicationContext
 import de.jeanlucmakiola.agendula.data.tasks.TasksContract.Instances
 import de.jeanlucmakiola.agendula.data.tasks.TasksContract.Lists
@@ -52,14 +55,47 @@ class AndroidTasksDataSource @Inject constructor(
     // --- reads ----------------------------------------------------------------
 
     override fun taskLists(): List<TaskList> {
-        val uri = TasksContract.listsUri(authority())
-        val sort = "${Lists.ACCOUNT_NAME}, ${Lists.NAME}"
-        return resolver.query(uri, TaskProjections.LISTS, null, null, sort)?.use { c ->
+        val authority = authority()
+        val lists = resolver.query(TasksContract.listsUri(authority), TaskProjections.LISTS, null, null, null)?.use { c ->
             val reader = CursorColumnReader(c)
-            buildList { while (c.moveToNext()) add(TaskMapper.taskList(reader)) }
-        } ?: emptyList()
+            buildList<TaskList> { while (c.moveToNext()) add(TaskMapper.taskList(reader)) }
+        } ?: return emptyList()
+        val order = synchronized(listOrderLock) {
+            val stored = storedListOrder(authority)
+            val known = lists.mapTo(HashSet()) { it.id }
+            // A list gone from the provider takes its position along, as its id may be
+            // reused — but not on an empty answer, which is as likely a provider mid-update.
+            if (lists.isNotEmpty() && stored.keys.any { it !in known }) {
+                stored.filterKeys { it in known }.also { storeListOrder(authority, it) }
+            } else {
+                stored
+            }
+        }
+        return ListOrder.sorted(lists, order)
     }
 
+    /** The order Agendula keeps for [authority]'s lists. */
+    internal fun storedListOrder(authority: String = authority()): Map<Long, Int> =
+        ListOrder.decode(listOrder.getString(authority, null))
+
+    private val listOrder get() = context.getSharedPreferences(LIST_ORDER, Context.MODE_PRIVATE)
+
+    private fun storeListOrder(authority: String, order: Map<Long, Int>) {
+        listOrder.edit().putString(authority, ListOrder.encode(order)).commit()
+    }
+
+    /** Kept by Agendula, as the provider's lists table has no order of its own. */
+    override fun reorderLists(listIds: List<Long>) {
+        val authority = authority()
+        synchronized(listOrderLock) {
+            storeListOrder(authority, ListOrder.reordered(taskLists().map { it.id }, listIds))
+        }
+        // Wakes the observers on the lists without asking any sync adapter to upload: nothing changed in the provider.
+        runCatching { resolver.notifyChange(TasksContract.listsUri(authority), null, 0) }
+    }
+
+    private val listOrderLock = Any()
+
     override fun tasks(query: TaskQuery): List<Task> {
         val clauses = mutableListOf<String>()
         val args = mutableListOf<String>()
@@ -73,14 +109,22 @@ class AndroidTasksDataSource @Inject constructor(
     }
 
     override fun task(taskId: Long): Task? {
-        val rows = queryInstances("${Instances.TASK_ID} = ?", arrayOf(taskId.toString()))
-        return rows.firstOrNull { it.distanceFromCurrent == 0 } ?: rows.firstOrNull()
+        roundLegacyAnchors()
+        val rows = instanceRows(resolver.query(TasksContract.instancesUri(authority()), null, "${Instances.TASK_ID} = ?", arrayOf(taskId.toString()), null))
+        return (rows.firstOrNull { it.task.distanceFromCurrent == 0 } ?: rows.firstOrNull())?.let { enrich(listOf(it)).single() }
     }
 
     override fun task(taskId: Long, occurrenceStart: Instant?): Task? {
         if (occurrenceStart == null) return task(taskId)
-        val rows = queryInstances("${Instances.TASK_ID} = ?", arrayOf(taskId.toString()))
-        return rows.firstOrNull { it.occurrenceStart == occurrenceStart } ?: task(taskId)
+        roundLegacyAnchors()
+        val rows = instanceRows(queryOccurrences(taskId))
+        val chosen = rows.firstOrNull { it.anchor == occurrenceStart }
+            ?: rows.firstOrNull { it.anchor?.let { a -> sameSecond(a, occurrenceStart) } == true }
+        if (chosen != null) return enrich(listOf(chosen)).single()
+        tickedCopy(taskId, occurrenceStart)?.let { copy -> return task(copy)?.copy(occurrenceStart = occurrenceStart) }
+        // A plain task has one occurrence, whatever it was addressed by. A series
+        // answers null rather than another occurrence a write would then land on.
+        return if (seriesRules(listOf(taskId)).isEmpty()) task(taskId) else null
     }
 
     override fun subtasks(parentTaskId: Long): List<Task> =
@@ -107,23 +151,74 @@ class AndroidTasksDataSource @Inject constructor(
         }
     } ?: emptyList()
 
+    /**
+     * All columns and no sort: the instances view differs across provider
+     * versions (tasks.org's has no `is_recurring`), the by-name mapper nulls what
+     * is missing, and the repository re-sorts.
+     */
     private fun queryInstances(selection: String?, args: Array<String>?): List<Task> {
-        val uri = TasksContract.instancesUri(authority())
-        // projection = null (all columns): the instances view differs across
-        // provider versions (e.g. tasks.org's bundled OpenTasks has no
-        // `is_recurring` / `distance_from_current`). The by-name mapper reads
-        // whatever is present and nulls the rest; the repository re-sorts, so we
-        // don't depend on a provider sort column either.
-        val tasks = resolver.query(uri, null, selection, args, null)
-            ?.use { c ->
-                val reader = CursorColumnReader(c)
-                buildList { while (c.moveToNext()) add(TaskMapper.task(reader)) }
-            } ?: emptyList()
-        if (tasks.isEmpty()) return tasks
+        roundLegacyAnchors()
+        return instanceTasks(resolver.query(TasksContract.instancesUri(authority()), null, selection, args, null))
+    }
+
+    /** Maps [cursor]'s instance rows, with the rule each series keeps on its `tasks` row, and their tags. */
+    private fun instanceTasks(cursor: Cursor?): List<Task> = enrich(instanceRows(cursor))
+
+    /** [cursor]'s instance rows, before their rules and tags are looked up. */
+    private fun instanceRows(cursor: Cursor?): List<InstanceRow> {
+        var flagged = false
+        val rows = cursor?.use { c ->
+            flagged = c.getColumnIndex(Instances.IS_RECURRING) >= 0
+            val reader = CursorColumnReader(c)
+            buildList { while (c.moveToNext()) add(TaskMapper.instance(reader)) }
+        }.orEmpty()
+        // Where the provider flags recurrence, only flagged rows need their rule looked up.
+        return if (flagged) rows.map { if (it.task.isRecurring) it else it.copy(ruleOwner = NO_RULE) } else rows
+    }
+
+    /** [rows] completed with the rule each series keeps on its `tasks` row, and their tags. */
+    private fun enrich(rows: List<InstanceRow>): List<Task> {
+        if (rows.isEmpty()) return emptyList()
+        val rules = seriesRules(rows.map { it.ruleOwner }.filter { it != NO_RULE })
+        val tasks = rows.map { TaskMapper.withRule(it, rules[it.ruleOwner]) }
         val tags = categories(tasks.map { it.taskId })
         return tasks.map { task -> tags[task.taskId]?.let { task.copy(tags = it) } ?: task }
     }
 
+    /** The recurrence of each of [taskIds] that is a series, keyed by task id. */
+    private fun seriesRules(taskIds: Collection<Long>): Map<Long, SeriesRule> = queryByIds(
+        TasksContract.tasksUri(authority()),
+        arrayOf(Tasks.ID, Tasks.RRULE, Tasks.RDATE),
+        Tasks.ID,
+        taskIds,
+        filter = "${Tasks.RRULE} IS NOT NULL OR ${Tasks.RDATE} IS NOT NULL",
+    ) { r -> r.getLong(Tasks.ID)?.let { id -> TaskMapper.seriesRule(r)?.let { id to it } } }.toMap()
+
+    /**
+     * Rows of [uri] whose [idColumn] is one of [ids], in chunks that stay under
+     * SQLite's bound-argument limit, each mapped by [read] (null rows dropped).
+     */
+    private fun <T : Any> queryByIds(
+        uri: Uri,
+        projection: Array<String>,
+        idColumn: String,
+        ids: Collection<Long>,
+        filter: String? = null,
+        filterArgs: List<String> = emptyList(),
+        sortOrder: String? = null,
+        read: (ColumnReader) -> T?,
+    ): List<T> = ids.distinct().chunked(MAX_SQL_ARGS - filterArgs.size).flatMap { chunk ->
+        val selection = buildString {
+            append("$idColumn IN (${chunk.joinToString(",") { "?" }})")
+            if (filter != null) append(" AND ($filter)")
+        }
+        val args = (chunk.map(Long::toString) + filterArgs).toTypedArray()
+        resolver.query(uri, projection, selection, args, sortOrder)?.use { c ->
+            val reader = CursorColumnReader(c)
+            buildList { while (c.moveToNext()) read(reader)?.let(::add) }
+        } ?: emptyList()
+    }
+
     override fun exportTasks(listId: Long): List<ExportTask> {
         // projection = null for the same reason queryInstances uses it: the tasks
         // table's shape varies across provider versions, and the by-name mapper
@@ -194,7 +289,7 @@ class AndroidTasksDataSource @Inject constructor(
     }
 
     override fun updateSeries(seriesId: Long, occurrenceStart: Instant, form: TaskForm, reminders: List<TaskReminder>?) {
-        val occurrence = task(seriesId, occurrenceStart)
+        val occurrence = occurrence(seriesId, occurrenceStart) { TaskMapper.instance(it).task } ?: task(seriesId)
         val anchors = resolver.query(
             taskUri(authority(), seriesId), arrayOf(Tasks.DTSTART, Tasks.DUE), null, null, null,
         )?.use { c ->
@@ -202,14 +297,14 @@ class AndroidTasksDataSource @Inject constructor(
             val r = CursorColumnReader(c)
             r.getLong(Tasks.DTSTART) to r.getLong(Tasks.DUE)
         }
-        fun shifted(anchor: Long?, was: Instant?, now: Instant?): Long? = when {
-            now == null -> null
-            anchor == null || was == null -> now.toEpochMilliseconds()
-            else -> anchor + (now - was).inWholeMilliseconds
-        }
-        val values = TaskWriteMapper.seriesValues(form, ZoneId.systemDefault().id) +
-            (Tasks.DTSTART to shifted(anchors?.first, occurrence?.start, form.start)) +
-            (Tasks.DUE to shifted(anchors?.second, occurrence?.due, form.due))
+        val base = TaskWriteMapper.seriesValues(form, ZoneId.systemDefault().id)
+        // Without an anchor to shift, the form's own value stands — already pinned for an all-day series.
+        fun shifted(column: String, anchor: Long?, was: Instant?, now: Instant?): Any? =
+            if (now == null || anchor == null || was == null) base[column]
+            else providerMillis(anchor + (now - was).inWholeMilliseconds)
+        val values = base +
+            (Tasks.DTSTART to shifted(Tasks.DTSTART, anchors?.first, occurrence?.start, form.start)) +
+            (Tasks.DUE to shifted(Tasks.DUE, anchors?.second, occurrence?.due, form.due))
         val rows = resolver.update(taskUri(authority(), seriesId), values.toContentValues(), null, null)
         if (rows == 0) throw TaskWriteFailedException("update series $seriesId")
         setCategories(seriesId, form.tags)
@@ -253,9 +348,10 @@ class AndroidTasksDataSource @Inject constructor(
         if (!c.moveToFirst()) return@use null
         val r = CursorColumnReader(c)
         val anchor = r.getLong(Tasks.DTSTART) ?: r.getLong(Tasks.DUE) ?: return@use null
+        val rule = TaskMapper.seriesRule(r)
         RecurrenceSpec(
-            rrule = r.getString(Tasks.RRULE)?.ifBlank { null },
-            rdate = r.getString(Tasks.RDATE),
+            rrule = rule?.rrule,
+            rdate = rule?.rdate,
             exdate = r.getString(Tasks.EXDATE),
             anchor = Instant.fromEpochMilliseconds(anchor),
             isAllDay = r.getBoolean(Tasks.IS_ALLDAY),
@@ -290,7 +386,7 @@ class AndroidTasksDataSource @Inject constructor(
         buildList {
             while (c.moveToNext()) {
                 val original = r.getLong(Tasks.ORIGINAL_INSTANCE_TIME) ?: continue
-                if (original >= from.toEpochMilliseconds()) r.getLong(Tasks.ID)?.let(::add)
+                if (providerMillis(original) >= providerMillis(from.toEpochMilliseconds())) r.getLong(Tasks.ID)?.let(::add)
             }
         }
     } ?: emptyList()
@@ -302,7 +398,10 @@ class AndroidTasksDataSource @Inject constructor(
     }
 
     override fun updateInstance(taskId: Long, occurrenceStart: Instant, form: TaskForm, reminders: List<TaskReminder>?) {
-        if (reminders == null) return writeInstance(taskId, occurrenceStart, form)
+        val hit = instanceFor(taskId, occurrenceStart)
+            ?: throw TaskWriteFailedException("update instance $taskId@$occurrenceStart: no such occurrence")
+        hit.overrideId?.let { return updateOverride(it, form, reminders) }
+        if (reminders == null) return writeInstance(taskId, hit.instanceId, form)
         // ⚠️ Written to the series, and then put back. Forking an occurrence
         // copies the task's *current* properties onto the new override row, so
         // setting the alarm beforehand is the only way to carry it across — and
@@ -311,15 +410,26 @@ class AndroidTasksDataSource @Inject constructor(
         val seriesReminders = alarmRows(taskId).map { it.second }
         writeReminders(taskId, reminders)
         try {
-            writeInstance(taskId, occurrenceStart, form)
+            writeInstance(taskId, hit.instanceId, form)
         } finally {
             setReminders(taskId, seriesReminders)
         }
     }
 
-    private fun writeInstance(taskId: Long, occurrenceStart: Instant, form: TaskForm) {
-        val instanceId = instanceIdFor(taskId, occurrenceStart)
-            ?: throw TaskWriteFailedException("update instance $taskId@$occurrenceStart: no such occurrence")
+    /**
+     * An occurrence that already has its own row is written there directly. The
+     * fork-and-restore dance in [updateInstance] would put its old tags and
+     * reminders back over the new ones.
+     */
+    private fun updateOverride(overrideId: Long, form: TaskForm, reminders: List<TaskReminder>?) {
+        val values = TaskWriteMapper.instanceValues(form, ZoneId.systemDefault().id)
+        val rows = resolver.update(taskUri(authority(), overrideId), values.toContentValues(), null, null)
+        if (rows == 0) throw TaskWriteFailedException("update override $overrideId")
+        setCategories(overrideId, form.tags)
+        writeReminders(overrideId, reminders)
+    }
+
+    private fun writeInstance(taskId: Long, instanceId: Long, form: TaskForm) {
         val values = TaskWriteMapper.instanceValues(form, ZoneId.systemDefault().id)
         val uri = TasksContract.instanceUri(authority(), instanceId)
         // The fork copies the series' properties onto the override, so the tags
@@ -346,28 +456,147 @@ class AndroidTasksDataSource @Inject constructor(
         }
     }
 
+    /** An occurrence's instance row, and the override row behind it if it has one. */
+    private class InstanceHit(val instanceId: Long, val overrideId: Long?)
+
     /**
-     * The provider's instance row id for one occurrence.
+     * The provider's instance row for one occurrence.
      *
      * The seam addresses occurrences by `(taskId, occurrenceStart)`; writing
      * through the instances URI still needs the row id, so it is looked up here
-     * rather than carried around above the data layer. Selection is on `task_id`
-     * only — the anchor is matched in Kotlin because the column that holds it
-     * (`instance_original_time`) is missing on older provider schemas, where a
-     * WHERE clause naming it would throw instead of falling back.
+     * rather than carried around above the data layer.
      */
-    private fun instanceIdFor(taskId: Long, occurrenceStart: Instant): Long? {
-        val uri = TasksContract.instancesUri(authority())
-        val selection = "${Instances.TASK_ID} = ?"
-        return resolver.query(uri, null, selection, arrayOf(taskId.toString()), null)?.use { c ->
+    private fun instanceFor(taskId: Long, occurrenceStart: Instant): InstanceHit? {
+        roundLegacyAnchors()
+        return occurrence(taskId, occurrenceStart) { r ->
+            r.getLong(Tasks.ID)?.let { id ->
+                InstanceHit(id, r.getLong(Instances.TASK_ID)?.takeIf { r.getLong(Tasks.ORIGINAL_INSTANCE_ID) != null })
+            }
+        }
+    }
+
+    private fun instanceIdFor(taskId: Long, occurrenceStart: Instant): Long? = instanceFor(taskId, occurrenceStart)?.instanceId
+
+    /**
+     * [read] applied to the instance row of [taskId]'s occurrence at
+     * [occurrenceStart] — its own, or an override's that replaced it — matched in
+     * whole seconds, as the provider expands them. The anchor is matched in
+     * Kotlin because `instance_original_time` is missing on older schemas.
+     */
+    private fun <T> occurrence(taskId: Long, occurrenceStart: Instant, read: (ColumnReader) -> T): T? =
+        queryOccurrences(taskId)?.use { c ->
             val reader = CursorColumnReader(c)
+            // An exact match first: a row of the same second is a fallback, not a peer.
+            var near: Int? = null
             while (c.moveToNext()) {
-                if (TaskMapper.occurrenceAnchor(reader) == occurrenceStart) {
-                    return@use reader.getLong(Tasks.ID)
+                val anchor = TaskMapper.occurrenceAnchor(reader) ?: continue
+                if (anchor == occurrenceStart) return@use read(reader)
+                if (near == null && sameSecond(anchor, occurrenceStart)) near = c.position
+            }
+            near?.let { c.moveToPosition(it); read(reader) }
+        }
+
+    /**
+     * Instance rows of [taskId] and of the overrides standing in for its
+     * occurrences. A provider whose view lacks `original_instance_id` gets the
+     * task's own rows only, rather than a failed query.
+     */
+    private fun queryOccurrences(taskId: Long): Cursor? {
+        val authority = authority()
+        val uri = TasksContract.instancesUri(authority)
+        val id = taskId.toString()
+        if (ownRowsOnlyFor != authority) {
+            try {
+                return resolver.query(uri, null, OCCURRENCE_OF, arrayOf(id, id), null)
+            } catch (e: SQLiteException) {
+                rememberIfMissingColumn(authority, e)
+            } catch (e: IllegalArgumentException) {
+                rememberIfMissingColumn(authority, e)
+            }
+        }
+        return resolver.query(uri, null, "${Instances.TASK_ID} = ?", arrayOf(id), null)
+    }
+
+    /** Only a view without the column is remembered; any other failure falls back for this query alone. */
+    private fun rememberIfMissingColumn(authority: String, e: Exception) {
+        val message = e.message.orEmpty()
+        if (Tasks.ORIGINAL_INSTANCE_ID in message || "no such column" in message || "Invalid column" in message) {
+            ownRowsOnlyFor = authority
+        } else {
+            Log.w(TAG, "Occurrence query failed, reading the task's own rows", e)
+        }
+    }
+
+    /** The provider whose instances view turned out to lack `original_instance_id`. */
+    @Volatile
+    private var ownRowsOnlyFor: String? = null
+
+    @Volatile
+    private var anchorsRoundedFor: String? = null
+
+    private val roundingLock = Any()
+
+    /**
+     * Rounds the DTSTART/DUE a build before [providerMillis] stored with
+     * milliseconds, which the provider cannot move past a completed occurrence.
+     * Once per provider, before the first read: concurrent readers wait for it.
+     * Written as the rows' sync adapter, since the server only ever had whole
+     * seconds, so nothing is dirtied or re-uploaded. Overrides need nothing: the
+     * provider forks them at its own whole-second instance times.
+     *
+     * Never fails a read. A row the provider refuses is left as it was and tried
+     * again on the next start; only a pass that rounded everything is remembered.
+     */
+    private fun roundLegacyAnchors() {
+        val authority = authority()
+        if (anchorsRoundedFor == authority) return
+        synchronized(roundingLock) {
+            if (anchorsRoundedFor == authority) return
+            val prefs = context.getSharedPreferences(MIGRATIONS, Context.MODE_PRIVATE)
+            val key = "anchors_rounded:$authority"
+            val done = prefs.getBoolean(key, false) || runCatching { roundAll(authority) }
+                .onFailure { Log.w(TAG, "Rounding legacy anchors failed", it) }
+                .getOrDefault(false)
+            if (done) prefs.edit().putBoolean(key, true).apply()
+            anchorsRoundedFor = authority
+        }
+    }
+
+    /** True when every stale row was rounded. */
+    private fun roundAll(authority: String): Boolean {
+        val columns = listOf(Tasks.DTSTART, Tasks.DUE)
+        val ops = resolver.query(
+            TasksContract.tasksUri(authority),
+            arrayOf(Tasks.ID, Tasks.ACCOUNT_NAME, Tasks.ACCOUNT_TYPE) + columns,
+            // Only series: the provider moves nothing else, so nothing else needs rounding.
+            "(${columns.joinToString(" OR ") { "$it % 1000 != 0" }}) AND (${Tasks.RRULE} IS NOT NULL OR ${Tasks.RDATE} IS NOT NULL)",
+            null,
+            null,
+        )?.use { c ->
+            val r = CursorColumnReader(c)
+            buildList<ContentProviderOperation> {
+                while (c.moveToNext()) {
+                    val id = r.getLong(Tasks.ID) ?: continue
+                    val values = buildMap<String, Any?> {
+                        for (column in columns) {
+                            val millis = r.getLong(column) ?: continue
+                            if (providerMillis(millis) != millis) put(column, providerMillis(millis))
+                        }
+                    }
+                    val name = r.getString(Tasks.ACCOUNT_NAME)
+                    val type = r.getString(Tasks.ACCOUNT_TYPE)
+                    val uri = if (name.isNullOrEmpty() || type.isNullOrEmpty()) taskUri(authority, id)
+                    else TasksContract.asSyncAdapter(taskUri(authority, id), name, type)
+                    add(ContentProviderOperation.newUpdate(uri).withValues(values.toContentValues()).build())
                 }
             }
-            null
-        }
+        } ?: return false
+        if (ops.isEmpty()) return true
+        val batch = runCatching { resolver.applyBatch(authority, ArrayList(ops)) }.getOrNull()
+        if (batch != null) return batch.all { (it.count ?: 0) > 0 }
+        // One refused row fails the whole batch; row by row, the rest still go.
+        return ops.map { op -> runCatching { resolver.applyBatch(authority, arrayListOf(op)).single().count ?: 0 }.getOrDefault(0) > 0 }
+            .all { it }
     }
 
     override fun setReminders(taskId: Long, reminders: List<TaskReminder>) {
@@ -430,29 +659,19 @@ class AndroidTasksDataSource @Inject constructor(
     }
 
     /** Tags by task id for [taskIds], in stored order. */
-    private fun categories(taskIds: Collection<Long>): Map<Long, List<String>> {
-        val rows = taskIds.distinct().chunked(MAX_SQL_ARGS - 1).flatMap { chunk ->
-            val selection = "${Properties.MIMETYPE} = ? AND ${Properties.TASK_ID} IN (${chunk.joinToString(",") { "?" }})"
-            val args = (listOf(TasksContract.Category.MIMETYPE) + chunk.map(Long::toString)).toTypedArray()
-            resolver.query(
-                TasksContract.propertiesUri(authority()),
-                arrayOf(Properties.TASK_ID, TasksContract.Category.NAME),
-                selection,
-                args,
-                Properties.PROPERTY_ID,
-            )?.use { c ->
-                val reader = CursorColumnReader(c)
-                buildList {
-                    while (c.moveToNext()) {
-                        val id = reader.getLong(Properties.TASK_ID) ?: continue
-                        val name = reader.getString(TasksContract.Category.NAME) ?: continue
-                        add(id to name)
-                    }
-                }
-            } ?: emptyList()
-        }
-        return rows.groupBy({ it.first }, { it.second }).mapValues { normalizeTags(it.value) }
-    }
+    private fun categories(taskIds: Collection<Long>): Map<Long, List<String>> = queryByIds(
+        TasksContract.propertiesUri(authority()),
+        arrayOf(Properties.TASK_ID, TasksContract.Category.NAME),
+        Properties.TASK_ID,
+        taskIds,
+        filter = "${Properties.MIMETYPE} = ?",
+        filterArgs = listOf(TasksContract.Category.MIMETYPE),
+        sortOrder = Properties.PROPERTY_ID,
+    ) { r ->
+        val id = r.getLong(Properties.TASK_ID)
+        val name = r.getString(TasksContract.Category.NAME)
+        if (id == null || name == null) null else id to name
+    }.groupBy({ it.first }, { it.second }).mapValues { normalizeTags(it.value) }
 
     /**
      * Replaces [taskId]'s category rows in one batch, so a refused insert cannot
@@ -482,16 +701,21 @@ class AndroidTasksDataSource @Inject constructor(
     }
 
     override fun setCancelled(taskId: Long, cancelled: Boolean) {
+        if (!cancelled && movedBack(taskId, null)) return
         val rows = resolver.update(taskUri(authority(), taskId), cancelValues(cancelled).toContentValues(), null, null)
         if (rows == 0) throw TaskWriteFailedException("cancel task $taskId")
     }
 
     /** Through the instances URI, like [setCompletedInstance]. */
     override fun setCancelledInstance(taskId: Long, occurrenceStart: Instant, cancelled: Boolean) {
+        if (!cancelled && movedBack(taskId, occurrenceStart)) return
         val instanceId = instanceIdFor(taskId, occurrenceStart)
-            ?: return setCancelled(taskId, cancelled)
+            ?: return tickedCopy(taskId, occurrenceStart)?.let { setCancelled(it, cancelled) }
+                ?: plainUnlessSeries(taskId, occurrenceStart) { setCancelled(taskId, cancelled) }
         val uri = TasksContract.instanceUri(authority(), instanceId)
-        val rows = resolver.update(uri, cancelValues(cancelled).toContentValues(), null, null)
+        val rows = rememberingCopy(taskId, occurrenceStart, closing = cancelled) {
+            resolver.update(uri, cancelValues(cancelled).toContentValues(), null, null)
+        }
         if (rows == 0) throw TaskWriteFailedException("cancel instance $instanceId")
     }
 
@@ -501,6 +725,7 @@ class AndroidTasksDataSource @Inject constructor(
     )
 
     override fun setCompleted(taskId: Long, completed: Boolean) {
+        if (!completed && movedBack(taskId, null)) return
         val values = TaskWriteMapper.completionValues(completed, System.currentTimeMillis())
         val rows = resolver.update(taskUri(authority(), taskId), values.toContentValues(), null, null)
         if (rows == 0) throw TaskWriteFailedException("complete task $taskId")
@@ -508,22 +733,133 @@ class AndroidTasksDataSource @Inject constructor(
 
     /**
      * Through the instances URI, which is what makes the provider fork an override
-     * rather than close the series. No instance row for the anchor means the task
-     * is not a series after all — the plain write is then the right one.
+     * rather than close the series. No instance row for the anchor on a task that
+     * is not a series means the plain write is the right one.
      */
     override fun setCompletedInstance(taskId: Long, occurrenceStart: Instant, completed: Boolean) {
+        if (!completed && movedBack(taskId, occurrenceStart)) return
         val instanceId = instanceIdFor(taskId, occurrenceStart)
-            ?: return setCompleted(taskId, completed)
+            ?: return tickedCopy(taskId, occurrenceStart)?.let { setCompleted(it, completed) }
+                ?: plainUnlessSeries(taskId, occurrenceStart) { setCompleted(taskId, completed) }
         val values = TaskWriteMapper.completionValues(completed, System.currentTimeMillis())
         val uri = TasksContract.instanceUri(authority(), instanceId)
-        val rows = resolver.update(uri, values.toContentValues(), null, null)
+        val rows = rememberingCopy(taskId, occurrenceStart, closing = completed) {
+            resolver.update(uri, values.toContentValues(), null, null)
+        }
         if (rows == 0) throw TaskWriteFailedException("complete instance $instanceId")
     }
 
+    /**
+     * Runs [write], and when it closed an occurrence the provider then detached
+     * into a task of its own, remembers that copy, so the occurrence can still be
+     * read and reopened, and how the series moved, so reopening can move it back.
+     */
+    private fun rememberingCopy(taskId: Long, occurrenceStart: Instant, closing: Boolean, write: () -> Int): Int {
+        if (!closing) return write()
+        val authority = authority()
+        val lastId = runCatching { lastTaskId(authority) }.getOrNull() ?: return write()
+        val before = runCatching { seriesState(authority, taskId) }.getOrNull()
+        return write().also { rows ->
+            if (rows > 0) {
+                runCatching {
+                    detachedSince(authority, lastId)?.let { copy ->
+                        val after = before?.let { seriesState(authority, taskId) }?.takeIf { it != before }
+                        val tick = Tick(copy, taskId.takeIf { after != null }, before.takeIf { after != null }, after, System.currentTimeMillis())
+                        ticks.put(ticks.key(authority, taskId, occurrenceStart), tick)
+                    }
+                }.onFailure { Log.w(TAG, "Looking up the detached copy failed", it) }
+            }
+        }
+    }
+
+    /**
+     * Undoes the tick that left [taskId]'s occurrence at [occurrenceStart] — or,
+     * with no occurrence, the tick that left the copy [taskId]: the series goes
+     * back where it was and the copy goes, as long as the series is still where
+     * the tick moved it and the copy has nothing filed under it. False when it
+     * cannot, so the caller reopens the copy as a task of its own.
+     */
+    private fun movedBack(taskId: Long, occurrenceStart: Instant?): Boolean = runCatching {
+        val authority = authority()
+        val (key, tick) = occurrenceStart?.let { at -> ticks.key(authority, taskId, at).let { k -> ticks.get(k)?.let { k to it } } }
+            ?: ticks.forCopy(authority, taskId)
+            ?: return false
+        val series = tick.seriesId ?: return false
+        val before = tick.before ?: return false
+        if (seriesState(authority, series) != tick.after || subtaskIds(tick.copyId).isNotEmpty()) return false
+        val restore = before.filter { (column, value) -> tick.after?.get(column) != value }
+        val ops = arrayListOf(
+            ContentProviderOperation.newUpdate(taskUri(authority, series)).withValues(restore.toContentValues()).build(),
+            ContentProviderOperation.newDelete(taskUri(authority, tick.copyId)).build(),
+        )
+        resolver.applyBatch(authority, ops)
+        ticks.remove(key)
+        true
+    }.onFailure { Log.w(TAG, "Moving the series back failed", it) }.getOrDefault(false)
+
+    /** The columns a tick moves a series by, or null when [taskId] is no series. */
+    private fun seriesState(authority: String, taskId: Long): Map<String, Any?>? {
+        if (seriesRules(listOf(taskId)).isEmpty()) return null
+        return resolver.query(taskUri(authority, taskId), SERIES_STATE, null, null, null)?.use { c ->
+            if (!c.moveToFirst()) return@use null
+            SERIES_STATE.indices.associate { i ->
+                SERIES_STATE[i] to when (c.getType(i)) {
+                    Cursor.FIELD_TYPE_NULL -> null
+                    Cursor.FIELD_TYPE_INTEGER -> c.getLong(i)
+                    else -> c.getString(i)
+                }
+            }
+        }
+    }
+
+    private fun lastTaskId(authority: String): Long =
+        resolver.query(TasksContract.tasksUri(authority), arrayOf(Tasks.ID), null, null, "${Tasks.ID} DESC")
+            ?.use { c -> if (c.moveToFirst()) c.getLong(0) else 0L } ?: 0L
+
+    /** The one plain task the provider created after [afterId], or null when it was not exactly one. */
+    private fun detachedSince(authority: String, afterId: Long): Long? =
+        resolver.query(TasksContract.tasksUri(authority), null, "${Tasks.ID} > ?", arrayOf(afterId.toString()), null)
+            ?.use { c ->
+                val r = CursorColumnReader(c)
+                buildList {
+                    while (c.moveToNext()) {
+                        val plain = r.getString(Tasks.RRULE) == null && r.getString(Tasks.RDATE) == null &&
+                            r.getLong(Tasks.ORIGINAL_INSTANCE_ID) == null
+                        if (plain) r.getLong(Tasks.ID)?.let(::add)
+                    }
+                }
+            }?.singleOrNull()
+
+    private val ticks = TickedCopies { context.getSharedPreferences(TICKED_COPIES, Context.MODE_PRIVATE) }
+
+    /** The copy a tick left for [taskId]'s occurrence at [occurrenceStart], if one was remembered. */
+    private fun tickedCopy(taskId: Long, occurrenceStart: Instant): Long? =
+        ticks.get(ticks.key(authority(), taskId, occurrenceStart))?.copyId
+
+    /**
+     * [write] for a task that turned out not to be a series. A series whose
+     * occurrence was not found fails instead: the plain write would close all of it.
+     */
+    private fun plainUnlessSeries(taskId: Long, occurrenceStart: Instant, write: () -> Unit) {
+        if (seriesRules(listOf(taskId)).isNotEmpty()) {
+            throw TaskWriteFailedException("instance $taskId@$occurrenceStart: no such occurrence")
+        }
+        write()
+    }
+
     override fun deleteTask(taskId: Long) {
         resolver.delete(taskUri(authority(), taskId), null, null)
     }
 
+    /**
+     * Through its instance, which the provider turns into an EXDATE on the master,
+     * as the Room store does. Deleting the row alone leaves the master without one.
+     */
+    override fun deleteOverride(taskId: Long, occurrenceStart: Instant) {
+        val instanceId = instanceIdFor(taskId, occurrenceStart) ?: return deleteTask(taskId)
+        resolver.delete(TasksContract.instanceUri(authority(), instanceId), null, null)
+    }
+
     override fun createLocalList(name: String, color: Int): Long {
         val uri = TasksContract.asSyncAdapter(
             TasksContract.listsUri(authority()),
@@ -594,5 +930,25 @@ class AndroidTasksDataSource @Inject constructor(
     }
 }
 
+private const val LIST_ORDER = "provider_list_order"
+
+/** An instance row of the task, or of an override standing in for one of its occurrences. */
+private const val OCCURRENCE_OF = "${Instances.TASK_ID} = ? OR ${Tasks.ORIGINAL_INSTANCE_ID} = ?"
+
+private const val MIGRATIONS = "provider_migrations"
+
+private const val TICKED_COPIES = "provider_ticked_copies"
+
+/** What a tick changes on a series, and an undo changes back. */
+private val SERIES_STATE = arrayOf(
+    Tasks.DTSTART, Tasks.DUE, Tasks.DURATION, Tasks.RRULE, Tasks.RDATE, Tasks.EXDATE,
+    Tasks.STATUS, Tasks.COMPLETED, Tasks.PERCENT_COMPLETE,
+)
+
+private const val TAG = "AndroidTasksDataSource"
+
+/** No rule to look up: the provider says the row does not recur. */
+private const val NO_RULE = -1L
+
 /** SQLite's default bound-argument limit on older Android releases. */
 private const val MAX_SQL_ARGS = 999

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/ListOrder.kt

@@ -0,0 +1,36 @@
+package de.jeanlucmakiola.agendula.data.tasks
+
+import de.jeanlucmakiola.agendula.domain.TaskList
+
+/**
+ * Agendula's own order for an external provider's lists, which the provider has
+ * no column for. Stored as `id=position` pairs. A list without one counts as
+ * position 0, as a new list does in Agendula's own store.
+ */
+object ListOrder {
+
+    fun decode(raw: String?): Map<Long, Int> = raw.orEmpty().split(',').mapNotNull { pair ->
+        val (id, position) = pair.split('=').takeIf { it.size == 2 } ?: return@mapNotNull null
+        id.toLongOrNull()?.let { key -> position.toIntOrNull()?.let { key to it } }
+    }.toMap()
+
+    fun encode(order: Map<Long, Int>): String = order.entries.joinToString(",") { "${it.key}=${it.value}" }
+
+    /**
+     * Positions for every list in [shown] — the lists in their current order —
+     * after one account's [listIds] were rearranged: those take each other's
+     * slots in their new order, and every other list keeps its own.
+     */
+    fun reordered(shown: List<Long>, listIds: List<Long>): Map<Long, Int> {
+        val moved = listIds.iterator()
+        val moving = listIds.toSet()
+        val result = shown.map { if (it in moving && moved.hasNext()) moved.next() else it }.toMutableList()
+        moved.forEachRemaining { if (it !in result) result += it }
+        return result.withIndex().associate { it.value to it.index }
+    }
+
+    fun sorted(lists: List<TaskList>, order: Map<Long, Int>): List<TaskList> = lists.sortedWith(
+        compareBy<TaskList>({ it.accountName.lowercase() }, { order[it.id] ?: 0 }, { it.name.lowercase() })
+            .thenBy { it.name },
+    )
+}

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/ModeRoutingTasksDataSource.kt

@@ -67,6 +67,7 @@ class ModeRoutingTasksDataSource(
         active().setCancelledInstance(taskId, occurrenceStart, cancelled)
 
     override fun deleteTask(taskId: Long) = active().deleteTask(taskId)
+    override fun deleteOverride(taskId: Long, occurrenceStart: Instant) = active().deleteOverride(taskId, occurrenceStart)
     override fun createLocalList(name: String, color: Int): Long = active().createLocalList(name, color)
     override fun updateList(listId: Long, name: String, color: Int) = active().updateList(listId, name, color)
     override fun deleteList(listId: Long) = active().deleteList(listId)

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/TaskMapper.kt

@@ -13,20 +13,23 @@ import kotlin.time.Instant
 /** Maps a provider row (via [ColumnReader]) to a domain model. Pure + testable. */
 object TaskMapper {
 
-    fun task(r: ColumnReader): Task {
+    /**
+     * Maps an `instances` row, without its series' rule: the dmfs instances view
+     * (DB 22 and 23) masks `rrule`/`rdate` with `NULL`, so the row alone cannot
+     * tell a series from a one-off on tasks.org's bundled provider (DB 22), which
+     * also lacks `is_recurring`. [withRule] completes it with the rule read from
+     * the master's `tasks` row. An override names its master in
+     * `original_instance_id`.
+     */
+    fun instance(r: ColumnReader): InstanceRow {
         fun instant(name: String): Instant? =
             r.getLong(name)?.let { Instant.fromEpochMilliseconds(it) }
 
         val rowId = r.getLong(Tasks.ID) ?: 0L
-        // Derived from the rule columns rather than the `is_recurring` column
-        // alone: that column only exists from OpenTasks 1.4.0 (DB 23) and is
-        // absent on tasks.org's bundled provider (DB 22), where reading it
-        // would silently report every recurring task as one-off — and route
-        // its edits onto the series anchor.
-        val recurring = r.getString(Tasks.RRULE) != null ||
-            r.getString(Tasks.RDATE) != null ||
-            r.getBoolean(Instances.IS_RECURRING)
-        return Task(
+        val masterId = r.getLong(Tasks.ORIGINAL_INSTANCE_ID)
+        val recurring = masterId != null || r.getBoolean(Instances.IS_RECURRING)
+        val anchor = occurrenceAnchor(r)
+        val task = Task(
             taskId = r.getLong(Instances.TASK_ID) ?: rowId,
             listId = r.getLong(Tasks.LIST_ID) ?: 0L,
             title = r.getString(Tasks.TITLE).orEmpty(),
@@ -47,13 +50,31 @@ object TaskMapper {
             accountName = r.getString(Tasks.ACCOUNT_NAME),
             parentId = r.getLong(Tasks.PARENT_ID),
             isRecurring = recurring,
-            occurrenceStart = if (recurring) occurrenceAnchor(r) else null,
+            occurrenceStart = if (recurring) anchor else null,
             distanceFromCurrent = r.getInt(Instances.DISTANCE_FROM_CURRENT),
             created = instant(Tasks.CREATED),
             lastModified = instant(Tasks.LAST_MODIFIED),
-            seriesId = if (recurring) r.getLong(Instances.TASK_ID) ?: rowId else null,
-            recurrenceRule = r.getString(Tasks.RRULE),
+            seriesId = if (recurring) masterId ?: r.getLong(Instances.TASK_ID) ?: rowId else null,
         )
+        return InstanceRow(task, ruleOwner = masterId ?: task.taskId, anchor = anchor)
+    }
+
+    /** [row]'s task as an occurrence of the series [rule] belongs to, or as it is without one. */
+    fun withRule(row: InstanceRow, rule: SeriesRule?): Task {
+        if (rule == null) return row.task
+        return row.task.copy(
+            isRecurring = true,
+            occurrenceStart = row.task.occurrenceStart ?: row.anchor,
+            seriesId = row.task.seriesId ?: row.ruleOwner,
+            recurrenceRule = rule.rrule,
+        )
+    }
+
+    /** The rule columns of a `tasks` row, or null when it does not recur. */
+    fun seriesRule(r: ColumnReader): SeriesRule? {
+        val rrule = r.getString(Tasks.RRULE)?.ifBlank { null }
+        val rdate = r.getString(Tasks.RDATE)?.ifBlank { null }
+        return if (rrule == null && rdate == null) null else SeriesRule(rrule, rdate)
     }
 
     /**
@@ -119,3 +140,12 @@ object TaskMapper {
         owner = r.getString(Lists.OWNER),
     )
 }
+
+/** A series' recurrence as stored on its master row. */
+data class SeriesRule(val rrule: String?, val rdate: String?)
+
+/**
+ * An instance row's task before its series' rule is applied. [ruleOwner] is the
+ * task whose `tasks` row carries that rule: the master, for an override.
+ */
+data class InstanceRow(val task: Task, val ruleOwner: Long, val anchor: Instant?)

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/TaskWriteMapper.kt

@@ -17,6 +17,20 @@ private fun Instant.forAllDay(allDay: Boolean): Instant =
         Math.floorDiv(toEpochMilliseconds(), MILLIS_PER_DAY) * MILLIS_PER_DAY,
     )
 
+/**
+ * Whole seconds, as iCalendar stores times. The provider expands a series from
+ * second-precision instants, so a DTSTART carrying milliseconds never matches its
+ * own first occurrence: completing it detaches a copy but leaves the series where
+ * it was, and the same occurrence comes straight back.
+ */
+fun providerMillis(epochMillis: Long): Long = Math.floorDiv(epochMillis, 1000L) * 1000L
+
+/** [at] cut to its whole second, as [providerMillis] does. */
+fun wholeSecond(at: Instant): Instant = Instant.fromEpochMilliseconds(providerMillis(at.toEpochMilliseconds()))
+
+/** True when [a] and [b] fall in the same whole second. */
+fun sameSecond(a: Instant, b: Instant): Boolean = wholeSecond(a) == wholeSecond(b)
+
 /**
  * Turns a [TaskForm] / mutation into a name→value map. Pure (no ContentValues),
  * so it unit-tests on the JVM; [AndroidTasksDataSource] converts the map to
@@ -57,8 +71,8 @@ object TaskWriteMapper {
         // through DateTime.toAllDay(), which drops the time-of-day and resolves the
         // remaining date against UTC — so a local-midnight instant lands on the
         // previous day for anyone west of UTC. Pin all-day values to UTC midnight.
-        put(Tasks.DTSTART, form.start?.forAllDay(form.isAllDay)?.toEpochMilliseconds())
-        put(Tasks.DUE, form.due?.forAllDay(form.isAllDay)?.toEpochMilliseconds())
+        put(Tasks.DTSTART, form.start?.forAllDay(form.isAllDay)?.toEpochMilliseconds()?.let(::providerMillis))
+        put(Tasks.DUE, form.due?.forAllDay(form.isAllDay)?.toEpochMilliseconds()?.let(::providerMillis))
         // DUE and DURATION are mutually exclusive. The provider's Validating
         // processor evaluates the *merged* row (supplied values over the stored
         // ones), so writing DUE onto a task that already carries a DURATION throws

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/TasksDataSource.kt

@@ -124,13 +124,17 @@ interface TasksDataSource {
      *
      * Implementations fall back to [setCompleted] when the row turns out not to
      * be a series master — an override, or a plain task the caller happened to
-     * hand an anchor for — so the routing above cannot get this wrong.
+     * hand an anchor for. A series whose occurrence cannot be found fails with
+     * [TaskWriteFailedException] instead: the plain write would close all of it.
      */
     fun setCompletedInstance(taskId: Long, occurrenceStart: Instant, completed: Boolean)
 
     /** Cancel (or restore) one occurrence of a series; the counterpart of [setCompletedInstance]. */
     fun setCancelledInstance(taskId: Long, occurrenceStart: Instant, cancelled: Boolean)
     fun deleteTask(taskId: Long)
+
+    /** Delete the override [taskId] standing for one occurrence, excluding that occurrence from its series. */
+    fun deleteOverride(taskId: Long, occurrenceStart: Instant) = deleteTask(taskId)
     fun createLocalList(name: String, color: Int): Long
 
     /** Rename and recolour [listId]. */

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/TasksRepositoryImpl.kt

@@ -68,9 +68,11 @@ class TasksRepositoryImpl @Inject constructor(
 
     override fun taskDetail(taskId: Long, occurrenceStart: Instant?): Flow<TaskDetail?> = observing {
         dataSource.task(taskId, occurrenceStart)?.let { task ->
+            // A series' subtasks hang off its master, also when this occurrence is an override.
+            val owner = task.seriesId ?: resolvedRow(task, taskId)
             TaskDetail(
                 task = task,
-                subtasks = SeriesCollapse.visible(dataSource.subtasks(taskId)).filter { it.taskId != taskId },
+                subtasks = SeriesCollapse.visible(dataSource.subtasks(owner)).filter { it.taskId != owner },
                 parent = task.parentId?.takeIf { it > 0 }?.let { dataSource.task(it) },
             )
         }
@@ -172,7 +174,9 @@ class TasksRepositoryImpl @Inject constructor(
     ) = withContext(io) {
         // Re-read just before writing: it settles the conflict check *and* tells
         // us which URI to write through.
+        // An occurrence that is gone is not written through to its series.
         val current = dataSource.task(taskId, occurrenceStart)
+            ?: if (occurrenceStart != null) throw TaskWriteFailedException("update task $taskId@$occurrenceStart: gone") else null
         // Conflict-safe overwrite: bail if the provider's last_modified moved
         // since the form captured it (external sync / another app). A null
         // baseline means "force / overwrite anyway".
@@ -201,7 +205,7 @@ class TasksRepositoryImpl @Inject constructor(
         // Going through the occurrence forks an override instead, which takes
         // the reminders on its own row and leaves the series' alone.
         val instance = current?.takeIf { it.isRecurring }?.occurrenceStart
-        if (instance == null) dataSource.updateTask(taskId, form, reminders)
+        if (instance == null) dataSource.updateTask(resolvedRow(current, taskId), form, reminders)
         else dataSource.updateInstance(taskId, instance, form, reminders)
     }
 
@@ -233,14 +237,16 @@ class TasksRepositoryImpl @Inject constructor(
     override suspend fun deleteTask(taskId: Long, occurrenceStart: Instant?, scope: RecurringScope) =
         withContext(io) {
             val current = dataSource.task(taskId, occurrenceStart)
+            // An occurrence that is gone leaves nothing to delete; its series stays.
+            if (current == null && occurrenceStart != null) return@withContext
             val seriesId = current?.seriesId
             val occurrence = current?.occurrenceStart
-            if (seriesId == null || occurrence == null) return@withContext dataSource.deleteWithSubtasks(taskId)
+            if (seriesId == null || occurrence == null) return@withContext dataSource.deleteWithSubtasks(resolvedRow(current, taskId))
             when (scope) {
                 RecurringScope.ThisOccurrence ->
                     // An override row deletes itself into an EXDATE; a generated
                     // occurrence has no row to delete.
-                    if (current.taskId != seriesId) dataSource.deleteTask(current.taskId)
+                    if (current.taskId != seriesId) dataSource.deleteOverride(current.taskId, occurrence)
                     else dataSource.deleteOccurrence(seriesId, occurrence)
                 RecurringScope.ThisAndFollowing -> dataSource.deleteFollowing(seriesId, occurrence)
                 RecurringScope.AllOccurrences -> dataSource.deleteWithSubtasks(seriesId)
@@ -291,6 +297,13 @@ class TasksRepositoryImpl @Inject constructor(
             ticks.close()
         }
     }.flowOn(io)
+
+    /**
+     * The row a read for [taskId] resolved to — a past occurrence the series moved
+     * past is a task of its own — which is the one a write has to address.
+     */
+    private fun resolvedRow(resolved: Task?, taskId: Long): Long = resolved?.taskId ?: taskId
+
 }
 
 /** Subtask levels a duplicate follows; deeper than any real list, short of a cycle. */

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/TickedCopies.kt

@@ -0,0 +1,84 @@
+package de.jeanlucmakiola.agendula.data.tasks
+
+import android.content.SharedPreferences
+import org.json.JSONObject
+import kotlin.time.Instant
+
+/**
+ * What ticking a provider occurrence left: the copy the provider detached, and
+ * the series' columns before and after the tick, so an undo can move it back.
+ */
+internal class Tick(
+    val copyId: Long,
+    val seriesId: Long?,
+    val before: Map<String, Any?>?,
+    val after: Map<String, Any?>?,
+    val at: Long,
+)
+
+/** The provider keeps no record of its ticks, so they are kept here for [HORIZON_MS]. */
+internal class TickedCopies(private val prefs: () -> SharedPreferences) {
+
+    fun key(authority: String, taskId: Long, occurrenceStart: Instant) =
+        "$authority|$taskId|${wholeSecond(occurrenceStart).epochSeconds}"
+
+    fun get(key: String): Tick? = prefs().getString(key, null)?.let(::decode)
+
+    /** The tick that left [copyId], with its key. */
+    fun forCopy(authority: String, copyId: Long): Pair<String, Tick>? =
+        prefs().all.entries.firstNotNullOfOrNull { (key, value) ->
+            if (!key.startsWith("$authority|")) return@firstNotNullOfOrNull null
+            (value as? String)?.let(::decode)?.takeIf { it.copyId == copyId }?.let { key to it }
+        }
+
+    fun put(key: String, tick: Tick) {
+        val stale = prefs().all.filter { (_, value) ->
+            val at = (value as? String)?.let(::decode)?.at ?: 0L
+            tick.at - at > HORIZON_MS
+        }.keys
+        prefs().edit().apply {
+            stale.forEach(::remove)
+            putString(key, encode(tick))
+        }.apply()
+    }
+
+    fun remove(key: String) {
+        prefs().edit().remove(key).apply()
+    }
+
+    private fun encode(tick: Tick): String = JSONObject().apply {
+        put("copy", tick.copyId)
+        put("at", tick.at)
+        tick.seriesId?.let { put("series", it) }
+        tick.before?.let { put("before", columns(it)) }
+        tick.after?.let { put("after", columns(it)) }
+    }.toString()
+
+    private fun columns(values: Map<String, Any?>) = JSONObject().apply {
+        values.forEach { (column, value) -> put(column, value ?: JSONObject.NULL) }
+    }
+
+    private fun decode(raw: String): Tick? = runCatching {
+        val json = JSONObject(raw)
+        Tick(
+            copyId = json.getLong("copy"),
+            seriesId = if (json.has("series")) json.getLong("series") else null,
+            before = json.optJSONObject("before")?.let(::values),
+            after = json.optJSONObject("after")?.let(::values),
+            at = json.getLong("at"),
+        )
+    }.getOrNull()
+
+    private fun values(json: JSONObject): Map<String, Any?> = json.keys().asSequence().associateWith { column ->
+        when (val value = json.get(column)) {
+            JSONObject.NULL -> null
+            is Number -> value.toLong()
+            else -> value.toString()
+        }
+    }
+
+    companion object {
+        /** As long as the own store keeps a tick undoable. */
+        const val HORIZON_MS = 30L * 24 * 60 * 60 * 1000
+    }
+}

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/provider/ProviderWriter.kt

@@ -62,39 +62,33 @@ class ProviderWriter internal constructor(
     private var batchWrote = false
 
     /** Inserts one row and returns its published id. */
-    fun insert(table: ProviderTable, values: ProviderValues): Long = write {
-        when (table) {
-            ProviderTable.TASKS -> insertTask(values)
-            ProviderTable.PROPERTIES -> insertProperty(values)
-            else -> throw UnsupportedOperationException("${table.path} is read-only")
-        }
+    fun insert(table: ProviderTable, values: ProviderValues): Long = when (table) {
+        ProviderTable.TASKS -> write { insertTask(values) }
+        ProviderTable.PROPERTIES -> write { insertProperty(values) }
+        ProviderTable.LISTS, ProviderTable.INSTANCES -> readOnly(table)
     }
 
     /** Updates every row [selection] matches and returns how many changed. */
-    fun update(table: ProviderTable, values: ProviderValues, selection: String?, args: Array<out String>?): Int = write {
-        when (table) {
-            ProviderTable.TASKS -> {
-                val writable = ProviderTaskWriter.writable(table, values, WRITABLE_TASK_COLUMNS)
-                val matched = ids(table, selection, args)
-                (if (Tasks.LIST_ID in writable) parentsFirst(matched) else matched).count { updateTask(it, writable) }
-            }
-            ProviderTable.INSTANCES -> {
-                val writable = ProviderTaskWriter.writable(table, values, WRITABLE_TASK_COLUMNS)
-                instances(selection, args).count { updateInstance(it, writable) }
-            }
-            ProviderTable.PROPERTIES -> ids(table, selection, args).count { updateProperty(it, values) }
-            else -> throw UnsupportedOperationException("${table.path} is read-only")
+    fun update(table: ProviderTable, values: ProviderValues, selection: String?, args: Array<out String>?): Int = when (table) {
+        ProviderTable.TASKS -> write {
+            val writable = ProviderTaskWriter.writable(table, values, WRITABLE_TASK_COLUMNS)
+            val matched = ids(table, selection, args)
+            (if (Tasks.LIST_ID in writable) parentsFirst(matched) else matched).count { updateTask(it, writable) }
         }
+        ProviderTable.INSTANCES -> write {
+            val writable = ProviderTaskWriter.writable(table, values, WRITABLE_TASK_COLUMNS)
+            instances(selection, args).count { updateInstance(it, writable) }
+        }
+        ProviderTable.PROPERTIES -> write { ids(table, selection, args).count { updateProperty(it, values) } }
+        ProviderTable.LISTS -> readOnly(table)
     }
 
     /** Deletes every row [selection] matches and returns how many went. */
-    fun delete(table: ProviderTable, selection: String?, args: Array<out String>?): Int = write {
-        when (table) {
-            ProviderTable.TASKS -> ids(table, selection, args).count { deleteTask(it) }
-            ProviderTable.INSTANCES -> instances(selection, args).count { deleteInstance(it) }
-            ProviderTable.PROPERTIES -> ids(table, selection, args).count { deleteProperty(it) }
-            else -> throw UnsupportedOperationException("${table.path} is read-only")
-        }
+    fun delete(table: ProviderTable, selection: String?, args: Array<out String>?): Int = when (table) {
+        ProviderTable.TASKS -> write { ids(table, selection, args).count { deleteTask(it) } }
+        ProviderTable.INSTANCES -> write { instances(selection, args).count { deleteInstance(it) } }
+        ProviderTable.PROPERTIES -> write { ids(table, selection, args).count { deleteProperty(it) } }
+        ProviderTable.LISTS -> readOnly(table)
     }
 
     /** Runs a client's batch in one transaction, re-arming reminders once at the end. */
@@ -113,6 +107,9 @@ class ProviderWriter internal constructor(
         }
     }
 
+    /** Refused as read-only whichever store is active, before the store is even looked at. */
+    private fun readOnly(table: ProviderTable): Nothing = throw UnsupportedOperationException("${table.path} is read-only")
+
     private fun <T> write(block: () -> T): T = source.locked {
         check(resolver.mode() == StorageMode.OWN) {
             "Agendula is working on top of an external tasks provider; write to that one instead"
@@ -192,7 +189,8 @@ class ProviderWriter internal constructor(
         val current = database.tasks().entity(taskId)?.takeUnless { it.isDeleted } ?: return false
         requireWritableList(current.listId)
         // An override deletes itself into an EXDATE on its master.
-        if (current.masterId != null) store.deleteTask(taskId) else store.deleteWithSubtasks(taskId)
+        val occurrence = current.recurrenceId
+        if (current.masterId != null && occurrence != null) store.deleteOverride(taskId, occurrence) else store.deleteWithSubtasks(taskId)
         return true
     }
 

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/room/AdvancedOccurrence.kt

@@ -0,0 +1,131 @@
+package de.jeanlucmakiola.agendula.data.tasks.room
+
+import androidx.room.ColumnInfo
+import androidx.room.Dao
+import androidx.room.Entity
+import androidx.room.ForeignKey
+import androidx.room.Index
+import androidx.room.Insert
+import androidx.room.OnConflictStrategy
+import androidx.room.PrimaryKey
+import androidx.room.Query
+import de.jeanlucmakiola.agendula.domain.TaskStatus
+import kotlin.time.Instant
+
+/**
+ * How a series was moved past one of its occurrences (#137). It maps the
+ * occurrence to its completed copy for as long as the copy lives, and while
+ * [undoable] it also holds what reopening the copy needs to move the series
+ * back: the change, not the old series, so edits made since are kept. Local
+ * only: never part of a `VTODO`.
+ */
+@Entity(
+    tableName = "advanced_occurrences",
+    foreignKeys = [
+        ForeignKey(entity = TaskEntity::class, parentColumns = ["id"], childColumns = ["copy_id"], onDelete = ForeignKey.CASCADE),
+        ForeignKey(entity = TaskEntity::class, parentColumns = ["id"], childColumns = ["master_id"], onDelete = ForeignKey.CASCADE),
+    ],
+    indices = [Index(value = ["master_id", "anchor"])],
+)
+data class AdvancedOccurrenceEntity(
+    /** The completed copy of the occurrence. */
+    @PrimaryKey @ColumnInfo(name = "copy_id") val copyId: Long,
+    @ColumnInfo(name = "master_id") val masterId: Long,
+    @ColumnInfo(name = "anchor") val anchor: Instant,
+    /** The anchor the series moved to; it is only moved back while it still has it. */
+    @ColumnInfo(name = "advanced_to") val advancedTo: Instant,
+    @ColumnInfo(name = "prior_dtstart") val priorDtstart: Instant?,
+    @ColumnInfo(name = "prior_due") val priorDue: Instant?,
+    @ColumnInfo(name = "prior_status") val priorStatus: TaskStatus,
+    @ColumnInfo(name = "prior_percent_complete") val priorPercentComplete: Int?,
+    @ColumnInfo(name = "prior_completed_at") val priorCompletedAt: Instant?,
+    /** Occurrences `COUNT` lost to the move. */
+    @ColumnInfo(name = "spent") val spent: Int,
+    /** The `RRULE` the move left; once it differs, the series was reshaped and is not moved back. */
+    @ColumnInfo(name = "rule_after") val ruleAfter: String?,
+    /** The `EXDATE` occurrences the move dropped, as epoch millis, comma-separated. */
+    @ColumnInfo(name = "dropped_exdates", defaultValue = "") val droppedExdates: String = "",
+    /** The copy was an override before, and goes back to being one. */
+    @ColumnInfo(name = "from_override") val fromOverride: Boolean,
+    /** Cleared once the copy is edited, moved or reopened on its own: it is then a task of its own. */
+    @ColumnInfo(name = "undoable", defaultValue = "1") val undoable: Boolean = true,
+    /** Set once the moved series reached the server, which then holds the move itself. */
+    @ColumnInfo(name = "settled", defaultValue = "0") val settled: Boolean = false,
+) {
+    val dropped: List<Instant>
+        get() = droppedExdates.split(',').mapNotNull { it.toLongOrNull()?.let(Instant::fromEpochMilliseconds) }
+
+    companion object {
+        fun encode(instants: List<Instant>) = instants.joinToString(",") { it.toEpochMilliseconds().toString() }
+    }
+}
+
+/** A closed override detached along with an advance; [taskId] is null once that row is gone. */
+@Entity(
+    tableName = "advanced_detached",
+    foreignKeys = [
+        ForeignKey(
+            entity = AdvancedOccurrenceEntity::class,
+            parentColumns = ["copy_id"],
+            childColumns = ["copy_id"],
+            onDelete = ForeignKey.CASCADE,
+        ),
+        ForeignKey(entity = TaskEntity::class, parentColumns = ["id"], childColumns = ["task_id"], onDelete = ForeignKey.SET_NULL),
+    ],
+    indices = [Index(value = ["copy_id"]), Index(value = ["task_id"])],
+)
+data class AdvancedDetachedEntity(
+    @PrimaryKey(autoGenerate = true) @ColumnInfo(name = "id") val id: Long = 0,
+    @ColumnInfo(name = "copy_id") val copyId: Long,
+    @ColumnInfo(name = "anchor") val anchor: Instant,
+    @ColumnInfo(name = "task_id") val taskId: Long?,
+)
+
+@Dao
+interface AdvancedOccurrenceDao {
+    @Insert(onConflict = OnConflictStrategy.REPLACE)
+    fun insert(row: AdvancedOccurrenceEntity)
+
+    @Insert
+    fun insertDetached(rows: List<AdvancedDetachedEntity>)
+
+    @Query("SELECT * FROM advanced_occurrences WHERE copy_id = :copyId")
+    fun forCopy(copyId: Long): AdvancedOccurrenceEntity?
+
+    @Query("SELECT * FROM advanced_detached WHERE copy_id = :copyId")
+    fun detachedFor(copyId: Long): List<AdvancedDetachedEntity>
+
+    @Query("SELECT * FROM advanced_occurrences WHERE master_id = :masterId")
+    fun forMaster(masterId: Long): List<AdvancedOccurrenceEntity>
+
+    /** The record for [masterId]'s occurrence within the second starting at [secondStart]. */
+    @Query(
+        "SELECT * FROM advanced_occurrences WHERE master_id = :masterId " +
+            "AND anchor >= :secondStart AND anchor < :secondStart + 1000",
+    )
+    fun at(masterId: Long, secondStart: Instant): AdvancedOccurrenceEntity?
+
+    @Query("DELETE FROM advanced_occurrences WHERE copy_id = :copyId")
+    fun delete(copyId: Long)
+
+    @Query("UPDATE advanced_occurrences SET undoable = 0 WHERE copy_id = :copyId")
+    fun keepAsOwnTask(copyId: Long)
+
+    @Query("UPDATE advanced_occurrences SET settled = 1 WHERE master_id IN (:masterIds)")
+    fun settle(masterIds: List<Long>)
+
+    /** Settles [masterId]'s moves to before [until], the anchor the server now holds. */
+    @Query("UPDATE advanced_occurrences SET settled = 1 WHERE master_id = :masterId AND advanced_to < :until")
+    fun settleUpTo(masterId: Long, until: Instant)
+
+    /** Records of ticks made before [before]: too old to undo. Counted from the tick, not the occurrence, which may be long overdue. */
+    @Query(
+        "DELETE FROM advanced_occurrences WHERE master_id = :masterId " +
+            "AND copy_id IN (SELECT id FROM tasks WHERE created_at < :before)",
+    )
+    fun pruneTickedBefore(masterId: Long, before: Instant)
+
+    /** [taskId], once detached with a tick, has a life of its own and no longer returns to the series. */
+    @Query("UPDATE advanced_detached SET task_id = NULL WHERE task_id = :taskId")
+    fun unlinkDetached(taskId: Long)
+}

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/room/RoomTaskMapper.kt

@@ -4,6 +4,7 @@ import de.jeanlucmakiola.agendula.domain.LocalAccount
 import de.jeanlucmakiola.agendula.domain.Task
 import de.jeanlucmakiola.agendula.domain.TaskList
 import de.jeanlucmakiola.agendula.domain.export.ExportTask
+import de.jeanlucmakiola.agendula.domain.isClosed
 import de.jeanlucmakiola.agendula.domain.priorityFromICal
 import de.jeanlucmakiola.agendula.domain.recurrence.RecurrenceSpec
 import kotlin.time.Instant
@@ -100,6 +101,10 @@ object RoomTaskMapper {
 val TaskEntity.isRecurring: Boolean
     get() = !rrule.isNullOrBlank() || !rdate.isNullOrBlank()
 
+/** Completed or cancelled. */
+val TaskEntity.isClosed: Boolean
+    get() = status.isClosed
+
 /**
  * The series anchor: `DTSTART` when present, else `DUE`. A `VTODO` may carry only
  * a due date, and RFC 5545 then anchors the recurrence on it — matching how the

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/room/RoomTasksDataSource.kt

@@ -6,13 +6,17 @@ import de.jeanlucmakiola.agendula.domain.TaskReminder
 import de.jeanlucmakiola.agendula.domain.TaskReminderOrder
 import de.jeanlucmakiola.agendula.data.tasks.TaskWriteFailedException
 import de.jeanlucmakiola.agendula.data.tasks.TasksDataSource
+import de.jeanlucmakiola.agendula.data.tasks.sameSecond
+import de.jeanlucmakiola.agendula.data.tasks.wholeSecond
 import de.jeanlucmakiola.agendula.domain.Task
 import de.jeanlucmakiola.agendula.domain.TaskForm
 import de.jeanlucmakiola.agendula.domain.TaskList
 import de.jeanlucmakiola.agendula.domain.TaskStatus
 import de.jeanlucmakiola.agendula.domain.export.ExportTask
+import de.jeanlucmakiola.agendula.domain.normalizeTags
 import de.jeanlucmakiola.agendula.domain.recurrence.ExpansionWindow
 import de.jeanlucmakiola.agendula.domain.recurrence.RecurrenceExpander
+import de.jeanlucmakiola.agendula.domain.recurrence.RecurrenceSpec
 import java.time.ZoneId
 import java.util.UUID
 import javax.inject.Inject
@@ -28,6 +32,12 @@ private val WINDOW_FORWARD = 730.days
 /** Residue lines that describe the old series and must not follow the new one. */
 private val SERIES_LINES = listOf("EXDATE", "RDATE", "RRULE", "RECURRENCE-ID", "UID")
 
+/** How long a record keeps mapping a ticked occurrence to its copy. */
+private val RECORD_HORIZON = 30.days
+
+/** Only bounds the search: the occurrence count is what stops expansion, so no real gap is cut off. */
+private val SERIES_LOOKAHEAD = (366 * 1000).days
+
 /** Enough to count every occurrence before a split point without bounding a real series. */
 internal const val SPLIT_COUNT_CEILING = 100_000
 
@@ -103,6 +113,7 @@ class RoomTasksDataSource @Inject constructor(
     private val tasks get() = database.tasks()
     private val lists get() = database.taskLists()
     private val alarms get() = database.alarms()
+    private val advanced get() = database.advancedOccurrences()
 
     // --- reads ----------------------------------------------------------------
 
@@ -130,9 +141,13 @@ class RoomTasksDataSource @Inject constructor(
         if (occurrenceStart == null) return task(taskId)
         val row = tasks.task(taskId) ?: return null
         if (row.task.recurrenceId != null || !row.task.isRecurring) return task(taskId)
-        return occurrencesOf(row, tasks.overrides(taskId), clock.now())
-            .firstOrNull { it.occurrenceStart == occurrenceStart }
-            ?: task(taskId)
+        val occurrences = occurrencesOf(row, tasks.overrides(taskId), clock.now())
+        // Null rather than another occurrence: a write addressed to this one must not land on that.
+        return occurrences.firstOrNull { it.occurrenceStart == occurrenceStart }
+            ?: occurrences.firstOrNull { it.occurrenceStart?.let { a -> sameSecond(a, occurrenceStart) } == true }
+            ?: copyAt(row.task, occurrenceStart)?.let { copy ->
+                tasks.task(copy.id)?.let { RoomTaskMapper.task(it).copy(occurrenceStart = occurrenceStart) }
+            }
     }
 
     private fun overrideTask(row: TaskRow, anchor: Instant): Task =
@@ -230,10 +245,8 @@ class RoomTasksDataSource @Inject constructor(
      * reminders sync, a sync landing between the two would upload the task
      * without them, or with the ones it is about to lose.
      */
-    private fun writeReminders(taskId: Long, reminders: List<TaskReminder>?) {
-        if (reminders == null) return
-        alarms.syncForTask(taskId, reminders)
-    }
+    private fun writeReminders(taskId: Long, reminders: List<TaskReminder>?): Boolean =
+        reminders != null && alarms.syncForTask(taskId, reminders)
 
     private fun storedReminders(taskId: Long): List<TaskReminder> =
         alarms.forTask(taskId).map { it.asReminder() }.sortedWith(TaskReminderOrder)
@@ -241,7 +254,7 @@ class RoomTasksDataSource @Inject constructor(
     override fun updateTask(taskId: Long, form: TaskForm, reminders: List<TaskReminder>?) = tx {
         val current = tasks.entity(taskId) ?: throw TaskWriteFailedException("update task $taskId")
         val edited = TaskFormWriter.apply(current, form, clock.now(), zone())
-        writeReminders(taskId, reminders)
+        val remindersChanged = writeReminders(taskId, reminders)
         if (current.masterId == null && edited.listId != current.listId) {
             if (lists.exists(edited.listId) == 0) throw TaskWriteFailedException("move task: no list ${edited.listId}")
             move(current, TaskFormWriter.withRule(edited, form.rrule))
@@ -256,18 +269,23 @@ class RoomTasksDataSource @Inject constructor(
         // in both (the task query skips non-null `master_id`, and the override
         // query finds no master in B) while still uploading as part of A's
         // resource.
-        tasks.update(
+        saveRow(
+            current,
             if (current.masterId == null) {
                 TaskFormWriter.withRule(edited, form.rrule)
             } else {
                 edited.copy(listId = current.listId, parentId = current.parentId)
             },
+            changedBesides = remindersChanged,
         )
         writes.onWritten(setOf(current.listId))
     }
 
     /** See [TaskMove]: the rows move, and the old collection is left a tombstone. */
     private fun move(current: TaskEntity, edited: TaskEntity) {
+        // A copy moved elsewhere is a task of its own now.
+        advanced.keepAsOwnTask(current.id)
+        advanced.unlinkDetached(current.id)
         val plan = TaskMove.plan(
             edited = edited,
             previous = current,
@@ -323,11 +341,7 @@ class RoomTasksDataSource @Inject constructor(
                 return@tx
             }
             val now = clock.now()
-            // COUNT counts before EXDATE is applied (RFC 5545 §3.8.5.3).
-            val spent = RecurrenceExpander.expand(
-                spec.copy(exdate = null),
-                ExpansionWindow(from = anchor, until = occurrenceStart, maxOccurrences = SPLIT_COUNT_CEILING),
-            ).size
+            val spent = spentBefore(spec, occurrenceStart)
             val (start, due) = resolvedTimes(master, occurrenceStart)
             tasks.update(TaskFormWriter.endedBefore(master, occurrenceStart, now, zone()))
 
@@ -350,9 +364,7 @@ class RoomTasksDataSource @Inject constructor(
                 percentComplete = null,
                 completedAt = null,
                 // Recurrence lines belong to the old series; everything else carries over.
-                unknownProperties = master.unknownProperties?.lines()
-                    ?.filterNot { line -> SERIES_LINES.any { line.startsWith(it, ignoreCase = true) } }
-                    ?.joinToString("\r\n")?.ifBlank { null },
+                unknownProperties = seriesFreeText(master.unknownProperties),
             )
             val applied = TaskFormWriter.apply(base, form.copy(percentComplete = null), now, zone())
             val delta = applied.dtstart?.let { newStart -> start?.let { newStart - it } }
@@ -411,9 +423,12 @@ class RoomTasksDataSource @Inject constructor(
 
     private inline fun tx(crossinline block: () -> Unit) = database.runInTransaction(Runnable { block() })
 
-    private fun seriesMaster(seriesId: Long): TaskEntity {
-        val row = tasks.entity(seriesId) ?: throw TaskWriteFailedException("series $seriesId")
-        return row.masterId?.let { tasks.entity(it) } ?: row
+    private fun seriesMaster(seriesId: Long): TaskEntity = rowAndMaster(seriesId).second
+
+    /** [taskId]'s row, and its series' master — the row itself unless it is an override. */
+    private fun rowAndMaster(taskId: Long): Pair<TaskEntity, TaskEntity> {
+        val row = tasks.entity(taskId) ?: throw TaskWriteFailedException("series $taskId")
+        return row to (row.masterId?.let { tasks.entity(it) } ?: row)
     }
 
     /** The occurrence's own times: its override's if it has one, else generated. */
@@ -460,62 +475,354 @@ class RoomTasksDataSource @Inject constructor(
 
     override fun setReminders(taskId: Long, reminders: List<TaskReminder>) = tx {
         if (!alarms.syncForTask(taskId, reminders)) return@tx
+        advanced.keepAsOwnTask(taskId)
+        advanced.unlinkDetached(taskId)
         // The reminders are part of the VTODO now, so changing them is an edit.
         val current = tasks.entity(taskId) ?: return@tx
         tasks.update(current.copy(isDirty = true, lastModified = clock.now()))
         writes.onWritten(setOf(current.listId))
     }
 
-    override fun setCancelled(taskId: Long, cancelled: Boolean) {
+    /** Reopening the copy a ticked occurrence left behind moves its series back, as in [setCompletedInstance]. */
+    override fun setCancelled(taskId: Long, cancelled: Boolean) = tx {
         val current = tasks.entity(taskId) ?: throw TaskWriteFailedException("cancel task $taskId")
-        tasks.update(TaskFormWriter.cancelled(current, cancelled, clock.now()))
+        saveRow(current, TaskFormWriter.cancelled(current, cancelled, clock.now()))
         writes.onWritten(setOf(current.listId))
     }
 
-    override fun setCompleted(taskId: Long, completed: Boolean) {
+    /** Reopening the copy a ticked occurrence left behind moves its series back, as in [setCompletedInstance]. */
+    override fun setCompleted(taskId: Long, completed: Boolean) = tx {
         val current = tasks.entity(taskId) ?: throw TaskWriteFailedException("complete task $taskId")
-        tasks.update(TaskFormWriter.completed(current, completed, clock.now()))
+        saveRow(current, TaskFormWriter.completed(current, completed, clock.now()))
         writes.onWritten(setOf(current.listId))
     }
 
     /**
-     * Ticking one occurrence forks a `RECURRENCE-ID` override carrying the
-     * completion — the same model [updateInstance] writes. Writing the status onto
-     * the master instead would close the series: the master is what
-     * [TaskDao.tasks] filters on, so every occurrence, past and future, would
-     * leave every list at once.
+     * Ticking a series' first open occurrence moves the series on to its next
+     * open one and keeps a completed copy, as tasks.org and Apple Reminders do — a
+     * completed `RECURRENCE-ID` override alone leaves the series showing as open
+     * in most other clients (#137). Closed overrides between become copies too.
+     * Reopening the copy while the series is still where that left it moves it
+     * back. Any other occurrence forks an override carrying the completion, the
+     * same model [updateInstance] writes. Writing the status onto the master
+     * instead would close the series.
      */
-    override fun setCompletedInstance(taskId: Long, occurrenceStart: Instant, completed: Boolean) =
-        writeInstance(taskId, occurrenceStart, "complete") { TaskFormWriter.completed(it, completed, clock.now()) }
+    override fun setCompletedInstance(taskId: Long, occurrenceStart: Instant, completed: Boolean) = tx {
+        closeOccurrence(taskId, occurrenceStart, completed) { TaskFormWriter.completed(it, completed, clock.now()) }
+    }
 
-    /** Same override model as [setCompletedInstance]. */
-    override fun setCancelledInstance(taskId: Long, occurrenceStart: Instant, cancelled: Boolean) =
-        writeInstance(taskId, occurrenceStart, "cancel") { TaskFormWriter.cancelled(it, cancelled, clock.now()) }
+    /** Same model as [setCompletedInstance]. */
+    override fun setCancelledInstance(taskId: Long, occurrenceStart: Instant, cancelled: Boolean) = tx {
+        closeOccurrence(taskId, occurrenceStart, cancelled) { TaskFormWriter.cancelled(it, cancelled, clock.now()) }
+    }
 
-    /** Applies [write] to the override for [occurrenceStart], forking one if there is none yet. */
-    private fun writeInstance(
-        taskId: Long,
-        occurrenceStart: Instant,
-        what: String,
-        write: (TaskEntity) -> TaskEntity,
-    ) {
-        val master = tasks.entity(taskId) ?: throw TaskWriteFailedException("$what instance $taskId")
-        // Not a series master — an override, or a plain task the caller handed an
-        // anchor for. Either way this row *is* the occurrence.
+    private fun closeOccurrence(taskId: Long, occurrenceStart: Instant, closing: Boolean, close: (TaskEntity) -> TaskEntity) {
+        val (row, master) = rowAndMaster(taskId)
         if (master.recurrenceSpec() == null) {
-            tasks.update(write(master))
-            writes.onWritten(setOf(master.listId))
+            // A plain task, or the copy a ticked occurrence left: this row is the occurrence.
+            saveRow(row, close(row))
+            writes.onWritten(setOf(row.listId))
             return
         }
+        val override = if (row.masterId != null) row else tasks.override(master.id, occurrenceStart)
+        val anchor = override?.recurrenceId ?: occurrenceStart
+        val seriesStart = master.recurrenceAnchor ?: return
+        if (anchor < seriesStart && !sameSecond(anchor, seriesStart)) {
+            // The series has moved past it, so a tick has nothing left to close;
+            // a reopen goes to the copy that occurrence left.
+            when {
+                override != null -> writeInstance(master, override, anchor, close)
+                !closing -> copyAt(master, anchor)?.let { copy ->
+                    saveRow(copy, close(copy))
+                    writes.onWritten(setOf(copy.listId))
+                }
+            }
+            return
+        }
+        if (master.isClosed) {
+            // A series ends with its last occurrence; reopening that one reopens the
+            // series. An override reopens alone while other occurrences are closed
+            // only through the master, which reopening it would reopen too.
+            val reopensSeries = !closing && (override == null || !closedThroughMaster(master, anchor))
+            if (reopensSeries) closeSeries(master, override, close) else writeInstance(master, override, anchor, close)
+            return
+        }
+        if (!closing || !advance(master, anchor, override, close)) writeInstance(master, override, anchor, close)
+    }
 
-        tasks.override(taskId, occurrenceStart)?.let {
-            tasks.update(write(it))
-            writes.onWritten(setOf(master.listId))
-            return
+    /**
+     * Writes [edited] over [current]. A copy whose only change is that it
+     * reopens moves its series back instead; any other edit, [changedBesides]
+     * included, makes it a task of its own, and it no longer stands for the
+     * series' occurrence. So does any edit to an occurrence detached with it.
+     */
+    private fun saveRow(current: TaskEntity, edited: TaskEntity, changedBesides: Boolean = false) {
+        advanced.unlinkDetached(current.id)
+        val record = advanced.forCopy(current.id)?.takeIf { it.undoable }
+        if (record != null) {
+            val unchanged = !changedBesides && sameApartFromState(current, edited)
+            if (unchanged && current.isClosed && !edited.isClosed && reopenCopy(current, edited, record)) return
+            if (!unchanged || !edited.isClosed) advanced.keepAsOwnTask(current.id)
+        }
+        tasks.update(edited)
+    }
+
+    /**
+     * Nothing a user sees changed but its state — what saving a reopened copy
+     * from the editor amounts to. Compared as the editor writes them back:
+     * trimmed, blank as none, tags normalised.
+     */
+    private fun sameApartFromState(a: TaskEntity, b: TaskEntity): Boolean {
+        fun text(value: String?) = value?.trim()?.ifEmpty { null }
+        return text(a.title) == text(b.title) && text(a.description) == text(b.description) &&
+            text(a.location) == text(b.location) && text(a.url) == text(b.url) &&
+            a.priority == b.priority && a.dtstart == b.dtstart && a.due == b.due && a.isAllDay == b.isAllDay &&
+            a.listId == b.listId && a.parentId == b.parentId && a.rrule == b.rrule && a.rdate == b.rdate &&
+            normalizeTags(a.categories) == normalizeTags(b.categories) && a.color == b.color
+    }
+
+    /** Whether an occurrence of [master] other than [except] has no override, so its state is the master's. */
+    private fun closedThroughMaster(master: TaskEntity, except: Instant): Boolean {
+        val spec = master.recurrenceSpec() ?: return false
+        val overridden = tasks.overrides(master.id).mapNotNull { it.recurrenceId?.let(::wholeSecond) }.toSet()
+        return expandFrom(spec, maxOccurrences = overridden.size + 2)
+            .any { !sameSecond(it, except) && wholeSecond(it) !in overridden }
+    }
+
+    /** [close] applied to the master — and to [override], its last occurrence — which ends the series. */
+    private fun closeSeries(master: TaskEntity, override: TaskEntity?, close: (TaskEntity) -> TaskEntity) {
+        tasks.update(close(master))
+        override?.let { tasks.update(close(it)) }
+        writes.onWritten(setOf(master.listId))
+    }
+
+    /**
+     * Moves [master] past [anchor] when that is its first open occurrence; false
+     * when it is not, so the caller writes an override. Series with `RDATE`s keep
+     * the override model (anchoring on one would re-align the `RRULE`), and so do
+     * recurring subtasks, whose copies would pile up under the parent.
+     */
+    private fun advance(master: TaskEntity, anchor: Instant, override: TaskEntity?, close: (TaskEntity) -> TaskEntity): Boolean {
+        val rule = master.rrule?.takeIf { it.isNotBlank() } ?: return false
+        if (!master.rdate.isNullOrBlank() || master.parentId != null) return false
+        val spec = master.recurrenceSpec() ?: return false
+        val overrides = tasks.overrides(master.id).associateBy { it.recurrenceId?.epochSeconds }
+        fun closedAt(at: Instant) = overrides[at.epochSeconds]?.isClosed == true
+        val leading = expandFrom(spec, maxOccurrences = overrides.values.count { it.isClosed } + 2)
+        val index = leading.indexOfFirst { sameSecond(it, anchor) }
+        if (index < 0 || leading.take(index).any { !closedAt(it) }) return false
+        var nextIndex = index + 1
+        while (nextIndex < leading.size && closedAt(leading[nextIndex])) nextIndex++
+        val next = leading.getOrNull(nextIndex) ?: return true.also { closeSeries(master, override, close) }
+
+        val now = clock.now()
+        val zone = master.timezone ?: zone()
+        val detached = (leading.take(index) + leading.subList(index + 1, nextIndex)).map { at ->
+            at to detach(overrides.getValue(at.epochSeconds), now)
+        }
+        // Expansion drops the anchor's sub-second part; the series keeps it.
+        val newAnchor = next + (spec.anchor - wholeSecond(spec.anchor))
+        val spent = spentBefore(spec, next)
+        val (start, due) = occurrenceTimes(master, newAnchor)
+        val ruleAfter = TaskFormWriter.remainingCount(rule, spent)
+        tasks.update(
+            master.copy(
+                dtstart = start,
+                due = due,
+                rrule = ruleAfter,
+                exdate = TaskFormWriter.valuesFrom(master.exdate, newAnchor, zone),
+                status = TaskStatus.NEEDS_ACTION,
+                percentComplete = null,
+                completedAt = null,
+                lastModified = now,
+                isDirty = true,
+            ),
+        )
+        val copyId = if (override != null) {
+            detach(override, now, close)
+        } else {
+            val (copyStart, copyDue) = occurrenceTimes(master, anchor)
+            val id = tasks.insert(close(asCopy(master, now).copy(dtstart = copyStart, due = copyDue)))
+            alarms.copyToTask(master.id, id, sameResource = false)
+            id
+        }
+        advanced.at(master.id, wholeSecond(anchor))?.let { advanced.delete(it.copyId) }
+        advanced.pruneTickedBefore(master.id, now - RECORD_HORIZON)
+        advanced.insert(
+            AdvancedOccurrenceEntity(
+                copyId = copyId,
+                masterId = master.id,
+                anchor = anchor,
+                advancedTo = newAnchor,
+                priorDtstart = master.dtstart,
+                priorDue = master.due,
+                priorStatus = master.status,
+                priorPercentComplete = master.percentComplete,
+                priorCompletedAt = master.completedAt,
+                spent = spent,
+                ruleAfter = ruleAfter,
+                droppedExdates = AdvancedOccurrenceEntity.encode(TaskFormWriter.instantsBefore(master.exdate, newAnchor, zone)),
+                fromOverride = override != null,
+            ),
+        )
+        advanced.insertDetached(detached.map { (at, id) -> AdvancedDetachedEntity(copyId = copyId, anchor = at, taskId = id) })
+        writes.onWritten(setOf(master.listId))
+        return true
+    }
+
+    /** Occurrences `COUNT` has spent before [cut]: counted before `EXDATE` (RFC 5545 §3.8.5.3). */
+    private fun spentBefore(spec: RecurrenceSpec, cut: Instant): Int = RecurrenceExpander.expand(
+        spec.copy(exdate = null),
+        ExpansionWindow(from = wholeSecond(spec.anchor), until = cut, maxOccurrences = SPLIT_COUNT_CEILING),
+    ).size
+
+    /**
+     * Turns the override [row] into a task of its own, in place so its id keeps
+     * working: a new UID, unlinked from the series, a resource of its own — and
+     * so its alarms are fresh copies, not the series' ones.
+     */
+    private fun detach(row: TaskEntity, now: Instant, close: (TaskEntity) -> TaskEntity = { it }): Long {
+        tasks.update(close(asCopy(row, now).copy(id = row.id)))
+        refreshAlarms(row.id)
+        return row.id
+    }
+
+    private fun refreshAlarms(taskId: Long) {
+        val current = alarms.forTask(taskId)
+        alarms.deleteForTask(taskId)
+        current.forEach { alarms.insert(it.freshCopy(taskId)) }
+    }
+
+    /** [source] as a standalone task: no series fields, never uploaded. */
+    private fun asCopy(source: TaskEntity, now: Instant): TaskEntity = source.copy(
+        id = 0,
+        uid = UUID.randomUUID().toString(),
+        masterId = null,
+        recurrenceId = null,
+        href = null,
+        etag = null,
+        rrule = null,
+        rdate = null,
+        exdate = null,
+        createdAt = now,
+        sequence = 0,
+        isDeleted = false,
+        isDirty = true,
+        unknownProperties = seriesFreeText(source.unknownProperties),
+    )
+
+    /**
+     * Undoes [advance] for the copy [row] while the series still sits where it
+     * moved it under the same rule, and the copy is still just that — same list,
+     * nothing filed under it. The anchor goes back, `COUNT` gets back what it
+     * spent, and the dropped `EXDATE`s return in the list's own shape; any other
+     * edit made since stays. The occurrence takes [reopened]'s state: as a plain
+     * occurrence again, or as an override where that state is its own. The
+     * occurrences detached with it become overrides again, or are excluded where
+     * their row is gone or has a life of its own. False when it cannot, so the
+     * caller reopens the copy as a task of its own.
+     */
+    private fun reopenCopy(row: TaskEntity, reopened: TaskEntity, record: AdvancedOccurrenceEntity): Boolean {
+        val master = tasks.entity(record.masterId)?.takeIf {
+            !it.isDeleted && !it.isClosed && row.listId == it.listId && sameRule(it.rrule, record.ruleAfter) &&
+                it.recurrenceAnchor?.let { a -> sameSecond(a, record.advancedTo) } == true
+        }
+        if (master == null || tasks.subtasks(row.id).isNotEmpty()) return false
+        val now = clock.now()
+        val zone = master.timezone ?: zone()
+        val synced = lists.entity(master.listId)?.accountId != null
+        val (start, due) = occurrenceTimes(master, record.priorDtstart ?: record.priorDue ?: record.anchor)
+        var back = master.copy(
+            dtstart = start,
+            due = due,
+            rrule = master.rrule?.let { TaskFormWriter.remainingCount(it, -record.spent) },
+            status = if (master.status == TaskStatus.NEEDS_ACTION) record.priorStatus else master.status,
+            percentComplete = master.percentComplete ?: record.priorPercentComplete,
+            completedAt = master.completedAt ?: record.priorCompletedAt,
+            lastModified = now,
+            isDirty = true,
+        )
+        record.dropped.forEach { back = TaskFormWriter.excepting(back, it, now, zone, dirty = synced) }
+        // A detached row given a parent or subtasks since has a life of its own, and stays out.
+        fun returns(task: TaskEntity?) = task != null && !task.isDeleted && task.listId == master.listId &&
+            task.parentId == master.parentId && tasks.subtasks(task.id).isEmpty()
+        val detached = advanced.detachedFor(row.id).map { it.anchor to it.taskId?.let(tasks::entity) }
+        detached.forEach { (at, task) -> if (!returns(task)) back = TaskFormWriter.excepting(back, at, now, zone, dirty = synced) }
+        tasks.update(back)
+        detached.forEach { (at, task) -> if (returns(task)) toOverride(task!!, back, at, now) }
+        advanced.delete(row.id)
+        val plain = reopened.status == TaskStatus.NEEDS_ACTION && reopened.percentComplete == null
+        if (record.fromOverride || !plain) toOverride(reopened, back, record.anchor, now)
+        else deleteTask(row.id)
+        writes.onWritten(setOf(master.listId))
+        return true
+    }
+
+    /** The same `RRULE` however a server re-serialised it: parts in any order and case. */
+    private fun sameRule(a: String?, b: String?): Boolean {
+        fun parts(rule: String?) = rule?.trim()?.removePrefix("RRULE:")?.split(';')
+            ?.map { it.trim().uppercase() }?.filter { it.isNotEmpty() }?.toSet()
+        return parts(a) == parts(b)
+    }
+
+    /** [row], a detached occurrence, back as [master]'s override for [anchor]. */
+    private fun toOverride(row: TaskEntity, master: TaskEntity, anchor: Instant, now: Instant) {
+        val back = row.copy(
+            uid = master.uid,
+            masterId = master.id,
+            recurrenceId = anchor,
+            href = null,
+            etag = null,
+            isDirty = true,
+            lastModified = now,
+            unknownProperties = seriesFreeText(row.unknownProperties),
+        )
+        if (row.href == null) {
+            tasks.update(back)
+            refreshAlarms(row.id)
+        } else {
+            // Already uploaded as a resource of its own: that one goes, a fresh override comes.
+            val id = tasks.insert(back.copy(id = 0))
+            alarms.copyToTask(row.id, id, sameResource = false)
+            deleteTask(row.id)
+        }
+    }
+
+    /** The copy [advance] left for [master]'s occurrence at [anchor], if it is still there. */
+    private fun copyAt(master: TaskEntity, anchor: Instant): TaskEntity? =
+        advanced.at(master.id, wholeSecond(anchor))?.let { tasks.entity(it.copyId) }?.takeUnless { it.isDeleted }
+
+    /** The series' occurrences from its anchor on, at most [maxOccurrences]. */
+    private fun expandFrom(spec: RecurrenceSpec, maxOccurrences: Int): List<Instant> {
+        val from = wholeSecond(spec.anchor)
+        return RecurrenceExpander.expand(spec, ExpansionWindow(from = from, until = from + SERIES_LOOKAHEAD, maxOccurrences = maxOccurrences))
+    }
+
+    /**
+     * The residue without the series' own lines, as a new resource must not
+     * carry them. Lines inside a nested component (a `VALARM`'s `UID`) are its
+     * own and stay.
+     */
+    private fun seriesFreeText(unknown: String?): String? {
+        var depth = 0
+        return unknown?.lines()?.filter { line ->
+            when {
+                line.startsWith("BEGIN:", ignoreCase = true) -> { depth++; true }
+                line.startsWith("END:", ignoreCase = true) -> { depth--; true }
+                else -> depth > 0 || (line.isNotBlank() && SERIES_LINES.none { line.startsWith(it, ignoreCase = true) })
+            }
+        }?.joinToString("\r\n")?.ifBlank { null }
+    }
+
+    /** Applies [write] to [override], or to a new one forked from [master] for [occurrenceStart]. */
+    private fun writeInstance(master: TaskEntity, override: TaskEntity?, occurrenceStart: Instant, write: (TaskEntity) -> TaskEntity) {
+        if (override != null) {
+            tasks.update(write(override))
+        } else {
+            val (start, due) = occurrenceTimes(master, occurrenceStart)
+            val id = tasks.insert(write(newOverride(master, master.id, occurrenceStart, start, due)))
+            alarms.copyToTask(master.id, id, sameResource = true)
         }
-        val (start, due) = occurrenceTimes(master, occurrenceStart)
-        val id = tasks.insert(write(newOverride(master, taskId, occurrenceStart, start, due)))
-        alarms.copyToTask(taskId, id, sameResource = true)
         writes.onWritten(setOf(master.listId))
     }
 
@@ -608,7 +915,7 @@ class RoomTasksDataSource @Inject constructor(
                     if (lists.exists(edited.listId) == 0) throw TaskWriteFailedException("move task: no list ${edited.listId}")
                     move(current, edited)
                 }
-                else -> tasks.update(edited)
+                else -> saveRow(current, edited)
             }
         }
         writes.onWritten(setOf(current.listId, edited.listId))

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/room/TaskDao.kt

@@ -199,4 +199,8 @@ interface TaskDao {
      */
     @Query("UPDATE tasks SET href = :href, etag = :etag, is_dirty = 0 WHERE id IN (:taskIds)")
     fun markSynced(taskIds: List<Long>, href: String?, etag: String?): Int
+
+    /** Sets `is_dirty` again on [taskId] when it was written after [seen], its `last_modified` when uploaded. */
+    @Query("UPDATE tasks SET is_dirty = 1 WHERE id = :taskId AND last_modified IS NOT :seen")
+    fun dirtyIfChangedSince(taskId: Long, seen: Instant?)
 }

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/room/TaskFormWriter.kt

@@ -120,18 +120,25 @@ object TaskFormWriter {
     fun valuesBefore(values: String?, cut: Instant, zone: String): String? =
         mapValues(values, zone) { it.takeIf { at -> at < cut } }
 
+    /** The instants of an `EXDATE`/`RDATE` list before [cut]; values it cannot read are left out. */
+    fun instantsBefore(values: String?, cut: Instant, zone: String): List<Instant> =
+        parsed(values, zone).mapNotNull { it.second }.filter { it < cut }
+
     /** The values of an `EXDATE`/`RDATE` list at or after [cut]. */
     fun valuesFrom(values: String?, cut: Instant, zone: String): String? =
         mapValues(values, zone) { it.takeIf { at -> at >= cut } }
 
     private fun mapValues(values: String?, zone: String, map: (Instant) -> Instant?): String? {
-        val list = values?.split(',')?.map(String::trim)?.filter(String::isNotEmpty) ?: return values
-        return list.mapNotNull { raw ->
-            val at = readValue(raw, zone) ?: return@mapNotNull raw
-            map(at)?.let { writeValueLike(raw, it, zone) }
+        if (values == null) return null
+        return parsed(values, zone).mapNotNull { (raw, at) ->
+            if (at == null) raw else map(at)?.let { writeValueLike(raw, it, zone) }
         }.joinToString(",").ifEmpty { null }
     }
 
+    /** Each value of an `EXDATE`/`RDATE` list with its instant, or null where it cannot be read. */
+    private fun parsed(values: String?, zone: String): List<Pair<String, Instant?>> =
+        values?.split(',')?.map(String::trim)?.filter(String::isNotEmpty)?.map { it to readValue(it, zone) }.orEmpty()
+
     private fun readValue(raw: String, zone: String): Instant? = runCatching {
         val z = java.time.ZoneId.of(zone)
         val instant = when {

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/room/TaskListDao.kt

@@ -18,7 +18,7 @@ interface TaskListDao {
         """
         SELECT l.*, a.display_name AS account_display_name, a.label AS account_label
         FROM task_lists l LEFT JOIN accounts a ON a.id = l.account_id
-        ORDER BY a.display_name, l.sort_order, l.name
+        ORDER BY a.display_name COLLATE NOCASE, l.sort_order, l.name COLLATE NOCASE, l.name
         """
     )
     fun lists(): List<TaskListRow>
@@ -32,6 +32,10 @@ interface TaskListDao {
     )
     fun list(listId: Long): TaskListRow?
 
+    /** The highest position among device-only lists, -1 when there are none. */
+    @Query("SELECT COALESCE(MAX(sort_order), -1) FROM task_lists WHERE account_id IS NULL")
+    fun lastLocalSortOrder(): Int
+
     @Query("SELECT * FROM task_lists WHERE id = :listId")
     fun entity(listId: Long): TaskListEntity?
 

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/room/TasksDatabase.kt

@@ -9,7 +9,7 @@ import java.util.UUID
 import kotlin.time.Instant
 
 /**
- * Agendula's own task store. Four tables, designed from
+ * Agendula's own task store. Six tables, designed from
  * what the app actually reads and writes plus RFC 5545's `VTODO`.
  *
  * Schemas are exported to `app/schemas/` and committed, so a future version can
@@ -21,8 +21,10 @@ import kotlin.time.Instant
         TaskListEntity::class,
         TaskEntity::class,
         TaskAlarmEntity::class,
+        AdvancedOccurrenceEntity::class,
+        AdvancedDetachedEntity::class,
     ],
-    version = 4,
+    version = 5,
     exportSchema = true,
 )
 @TypeConverters(Converters::class)
@@ -31,6 +33,7 @@ abstract class TasksDatabase : RoomDatabase() {
     abstract fun tasks(): TaskDao
     abstract fun alarms(): TaskAlarmDao
     abstract fun accounts(): AccountDao
+    abstract fun advancedOccurrences(): AdvancedOccurrenceDao
 
     companion object {
         const val NAME = "agendula-tasks.db"
@@ -180,5 +183,26 @@ abstract class TasksDatabase : RoomDatabase() {
                 db.execSQL("ALTER TABLE accounts ADD COLUMN label TEXT")
             }
         }
+
+        /** v5 remembers how ticking an occurrence moved its series, so reopening it can move it back. */
+        val MIGRATION_4_5 = object : Migration(4, 5) {
+            override fun migrate(db: SupportSQLiteDatabase) {
+                db.execSQL(
+                    "CREATE TABLE IF NOT EXISTS `advanced_occurrences` (`copy_id` INTEGER NOT NULL, `master_id` INTEGER NOT NULL, `anchor` INTEGER NOT NULL, `advanced_to` INTEGER NOT NULL, `prior_dtstart` INTEGER, `prior_due` INTEGER, `prior_status` INTEGER NOT NULL, `prior_percent_complete` INTEGER, `prior_completed_at` INTEGER, `spent` INTEGER NOT NULL, `rule_after` TEXT, `dropped_exdates` TEXT NOT NULL DEFAULT '', `from_override` INTEGER NOT NULL, `undoable` INTEGER NOT NULL DEFAULT 1, `settled` INTEGER NOT NULL DEFAULT 0, PRIMARY KEY(`copy_id`), FOREIGN KEY(`copy_id`) REFERENCES `tasks`(`id`) ON UPDATE NO ACTION ON DELETE CASCADE , FOREIGN KEY(`master_id`) REFERENCES `tasks`(`id`) ON UPDATE NO ACTION ON DELETE CASCADE )",
+                )
+                db.execSQL(
+                    "CREATE INDEX IF NOT EXISTS `index_advanced_occurrences_master_id_anchor` ON `advanced_occurrences` (`master_id`, `anchor`)",
+                )
+                db.execSQL(
+                    "CREATE TABLE IF NOT EXISTS `advanced_detached` (`id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `copy_id` INTEGER NOT NULL, `anchor` INTEGER NOT NULL, `task_id` INTEGER, FOREIGN KEY(`copy_id`) REFERENCES `advanced_occurrences`(`copy_id`) ON UPDATE NO ACTION ON DELETE CASCADE , FOREIGN KEY(`task_id`) REFERENCES `tasks`(`id`) ON UPDATE NO ACTION ON DELETE SET NULL )",
+                )
+                db.execSQL(
+                    "CREATE INDEX IF NOT EXISTS `index_advanced_detached_copy_id` ON `advanced_detached` (`copy_id`)",
+                )
+                db.execSQL(
+                    "CREATE INDEX IF NOT EXISTS `index_advanced_detached_task_id` ON `advanced_detached` (`task_id`)",
+                )
+            }
+        }
     }
 }

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/transfer/ExternalImport.kt

@@ -159,7 +159,9 @@ class ExternalImport @Inject constructor(
         val taskDao = database.tasks()
         val alarmDao = database.alarms()
 
-        val listIds = snapshot.lists.associate { list ->
+        // After the device-only lists already there, which the copies join.
+        val after = listDao.lastLocalSortOrder() + 1
+        val listIds = snapshot.lists.withIndex().associate { (position, list) ->
             list.id to listDao.insert(
                 TaskListEntity(
                     name = list.name,
@@ -168,6 +170,7 @@ class ExternalImport @Inject constructor(
                     isVisible = list.isVisible,
                     isSynced = false,
                     owner = list.owner,
+                    sortOrder = after + position,
                 ),
             )
         }

app/src/main/java/de/jeanlucmakiola/agendula/domain/Models.kt

@@ -37,6 +37,9 @@ data class TaskList(
 
 enum class TaskStatus { NEEDS_ACTION, IN_PROCESS, COMPLETED, CANCELLED }
 
+/** Completed or cancelled. */
+val TaskStatus.isClosed: Boolean get() = this == TaskStatus.COMPLETED || this == TaskStatus.CANCELLED
+
 /** iCalendar priority, bucketed for the UI. */
 enum class Priority { NONE, LOW, MEDIUM, HIGH }
 
@@ -111,7 +114,7 @@ data class Task(
     val isOccurrence: Boolean get() = seriesId != null && occurrenceStart != null
 
     val isCompleted: Boolean get() = status == TaskStatus.COMPLETED
-    val isClosed: Boolean get() = status == TaskStatus.COMPLETED || status == TaskStatus.CANCELLED
+    val isClosed: Boolean get() = status.isClosed
     val isSubtask: Boolean get() = parentId != null && parentId > 0
     /** The task's own colour if set, else the list colour. */
     val effectiveColor: Int get() = taskColor ?: listColor

app/src/test/java/de/jeanlucmakiola/agendula/data/tasks/ListOrderTest.kt

@@ -0,0 +1,56 @@
+package de.jeanlucmakiola.agendula.data.tasks
+
+import com.google.common.truth.Truth.assertThat
+import de.jeanlucmakiola.agendula.domain.TaskList
+import org.junit.jupiter.api.Test
+
+class ListOrderTest {
+
+    private fun list(id: Long, name: String, account: String = "Radicale") = TaskList(
+        id = id, name = name, color = 0, accountName = account, accountType = "bitfire.at.davdroid",
+        isSynced = true, isVisible = true, owner = null,
+    )
+
+    @Test
+    fun `an order survives encoding`() {
+        val order = mapOf(3L to 0, 1L to 1, 2L to 2)
+        assertThat(ListOrder.decode(ListOrder.encode(order))).isEqualTo(order)
+    }
+
+    @Test
+    fun `nothing stored and garbage both decode to no order`() {
+        assertThat(ListOrder.decode(null)).isEmpty()
+        assertThat(ListOrder.decode("x=1,2,=3,4=y")).isEmpty()
+    }
+
+    @Test
+    fun `lists follow the stored order within their account (#136)`() {
+        val lists = listOf(list(1, "Groceries"), list(2, "Home"), list(3, "Work"), list(4, "Inbox", account = "Local"))
+        val order = ListOrder.reordered(listOf(4, 1, 2, 3), listOf(3, 1, 2))
+
+        assertThat(ListOrder.sorted(lists, order).map { it.id }).containsExactly(4L, 3L, 1L, 2L).inOrder()
+    }
+
+    @Test
+    fun `reordering one account leaves every other list in its slot`() {
+        // 7 and 8 are another account; 5 joined this one after the screen was drawn.
+        val order = ListOrder.reordered(listOf(7, 8, 1, 5, 2), listOf(2, 1))
+
+        assertThat(order).isEqualTo(mapOf(7L to 0, 8L to 1, 2L to 2, 5L to 3, 1L to 4))
+        assertThat(order.values.toSet()).hasSize(order.size)
+    }
+
+    @Test
+    fun `names sort without regard to case`() {
+        val lists = listOf(list(1, "Zeta"), list(2, "apple"))
+        assertThat(ListOrder.sorted(lists, emptyMap()).map { it.id }).containsExactly(2L, 1L).inOrder()
+    }
+
+    @Test
+    fun `a list without a position counts as the first, as in the own store`() {
+        val lists = listOf(list(1, "Zeta"), list(2, "Alpha"), list(3, "Mid"))
+        val order = mapOf(3L to 0, 1L to 1)
+
+        assertThat(ListOrder.sorted(lists, order).map { it.id }).containsExactly(2L, 3L, 1L).inOrder()
+    }
+}

app/src/test/java/de/jeanlucmakiola/agendula/data/tasks/TaskMapperTest.kt

@@ -7,6 +7,7 @@ import de.jeanlucmakiola.agendula.data.tasks.TasksContract.Tasks
 import de.jeanlucmakiola.agendula.domain.Priority
 import de.jeanlucmakiola.agendula.domain.TaskStatus
 import org.junit.jupiter.api.Test
+import kotlin.time.Instant
 
 class TaskMapperTest {
 
@@ -34,7 +35,7 @@ class TaskMapperTest {
             ),
         )
 
-        val task = TaskMapper.task(reader)
+        val task = mapTask(reader)
 
         assertThat(task.taskId).isEqualTo(7L)
         assertThat(task.listId).isEqualTo(3L)
@@ -52,7 +53,7 @@ class TaskMapperTest {
     @Test
     fun `an occurrence is identified by its recurrence-id anchor`() {
         fun occurrence(columns: Map<String, Any?>) =
-            TaskMapper.task(MapColumnReader(columns + (Tasks.RRULE to "FREQ=DAILY")))
+            mapTask(MapColumnReader(columns), SeriesRule(rrule = "FREQ=DAILY", rdate = null))
 
         // instance_original_time is the provider's own RECURRENCE-ID and wins.
         val anchored = occurrence(
@@ -76,7 +77,7 @@ class TaskMapperTest {
 
     @Test
     fun `a non-recurring task has no occurrence anchor and keys by task id`() {
-        val task = TaskMapper.task(
+        val task = mapTask(
             MapColumnReader(mapOf(Tasks.ID to 4L, Instances.INSTANCE_START to 500L)),
         )
         assertThat(task.occurrenceStart).isNull()
@@ -84,33 +85,81 @@ class TaskMapperTest {
     }
 
     @Test
-    fun `recurrence is detected from rrule when is_recurring is absent`() {
-        // tasks.org's bundled provider is DB 22 and has no `is_recurring` column;
-        // reading it alone would report the series as one-off and send its edits
-        // to the master row, re-anchoring the whole thing.
-        val task = TaskMapper.task(
-            MapColumnReader(mapOf(Tasks.ID to 1L, Tasks.RRULE to "FREQ=WEEKLY;BYDAY=MO")),
+    fun `a series is detected from its master rule when the view masks it (#135)`() {
+        // tasks.org (DB 22): rule masked in the view, no `is_recurring`.
+        val task = mapTask(
+            MapColumnReader(
+                mapOf(
+                    Tasks.ID to 90L,
+                    Instances.TASK_ID to 9L,
+                    Instances.INSTANCE_START to 1_000L,
+                    Instances.INSTANCE_ORIGINAL_TIME to 1_000L,
+                ),
+            ),
+            SeriesRule(rrule = "FREQ=DAILY", rdate = null),
         )
         assertThat(task.isRecurring).isTrue()
+        assertThat(task.seriesId).isEqualTo(9L)
+        assertThat(task.occurrenceStart).isEqualTo(Instant.fromEpochMilliseconds(1_000L))
+        assertThat(task.isOccurrence).isTrue()
+        assertThat(task.recurrenceRule).isEqualTo("FREQ=DAILY")
+    }
+
+    @Test
+    fun `the master rule fills the editor even where is_recurring exists`() {
+        // OpenTasks (DB 23) flags the row but masks the rule too.
+        val task = mapTask(
+            MapColumnReader(mapOf(Tasks.ID to 90L, Instances.TASK_ID to 9L, Instances.IS_RECURRING to 1)),
+            SeriesRule(rrule = "FREQ=WEEKLY;BYDAY=MO", rdate = null),
+        )
+        assertThat(task.recurrenceRule).isEqualTo("FREQ=WEEKLY;BYDAY=MO")
+    }
+
+    @Test
+    fun `a tasks row without a rule is no series`() {
+        assertThat(TaskMapper.seriesRule(MapColumnReader(mapOf(Tasks.ID to 1L, Tasks.RRULE to "")))).isNull()
+        assertThat(TaskMapper.seriesRule(MapColumnReader(mapOf(Tasks.ID to 1L, Tasks.RDATE to "20260720T090000Z"))))
+            .isEqualTo(SeriesRule(rrule = null, rdate = "20260720T090000Z"))
     }
 
     @Test
     fun `recurrence is detected from rdate alone`() {
-        val task = TaskMapper.task(
-            MapColumnReader(mapOf(Tasks.ID to 1L, Tasks.RDATE to "20260720T090000Z")),
+        val task = mapTask(
+            MapColumnReader(mapOf(Tasks.ID to 1L)),
+            SeriesRule(rrule = null, rdate = "20260720T090000Z"),
         )
         assertThat(task.isRecurring).isTrue()
     }
 
+    @Test
+    fun `an override belongs to its master's series`() {
+        // An edited occurrence is its own task row, without a rule of its own.
+        val task = mapTask(
+            MapColumnReader(
+                mapOf(
+                    Tasks.ID to 95L,
+                    Instances.TASK_ID to 12L,
+                    Tasks.ORIGINAL_INSTANCE_ID to 9L,
+                    Instances.INSTANCE_ORIGINAL_TIME to 2_000L,
+                ),
+            ),
+            SeriesRule(rrule = "FREQ=DAILY", rdate = null),
+        )
+        assertThat(task.taskId).isEqualTo(12L)
+        assertThat(task.seriesId).isEqualTo(9L)
+        assertThat(task.occurrenceStart).isEqualTo(Instant.fromEpochMilliseconds(2_000L))
+        assertThat(task.recurrenceRule).isEqualTo("FREQ=DAILY")
+    }
+
     @Test
     fun `a plain task is not recurring`() {
-        val task = TaskMapper.task(MapColumnReader(mapOf(Tasks.ID to 1L, Tasks.TITLE to "One-off")))
+        val task = mapTask(MapColumnReader(mapOf(Tasks.ID to 1L, Tasks.TITLE to "One-off")))
         assertThat(task.isRecurring).isFalse()
     }
 
     @Test
     fun `falls back to instance id when task_id missing, and list color when no task color`() {
-        val task = TaskMapper.task(
+        val task = mapTask(
             MapColumnReader(mapOf(Tasks.ID to 9L, Tasks.LIST_COLOR to 0x111111)),
         )
         assertThat(task.taskId).isEqualTo(9L)
@@ -140,3 +189,5 @@ class TaskMapperTest {
         assertThat(list.isLocal).isFalse()
     }
 }
+
+private fun mapTask(r: ColumnReader, rule: SeriesRule? = null) = TaskMapper.withRule(TaskMapper.instance(r), rule)

app/src/test/java/de/jeanlucmakiola/agendula/data/tasks/TaskWriteMapperTest.kt

@@ -102,14 +102,15 @@ class TaskWriteMapperTest {
     }
 
     @Test
-    fun `timed timestamps are written untouched`() {
+    fun `timed timestamps are kept to the second`() {
+        // The provider expands series in whole seconds.
         val at = Instant.fromEpochMilliseconds(1_784_412_345_678L)
         val values = TaskWriteMapper.taskValues(
             TaskForm(title = "Standup", listId = 1L, start = at, due = at),
             tzId = "Europe/Berlin",
         )
-        assertThat(values[Tasks.DTSTART]).isEqualTo(1_784_412_345_678L)
-        assertThat(values[Tasks.DUE]).isEqualTo(1_784_412_345_678L)
+        assertThat(values[Tasks.DTSTART]).isEqualTo(1_784_412_345_000L)
+        assertThat(values[Tasks.DUE]).isEqualTo(1_784_412_345_000L)
     }
 
     @Test

app/src/testFull/java/de/jeanlucmakiola/agendula/data/sync/SyncFakes.kt

@@ -76,11 +76,12 @@ class FakeStore(rows: List<TaskEntity> = emptyList()) : SyncStore {
         return id
     }
 
-    override fun markSynced(taskIds: List<Long>, href: String?, eTag: String?) {
-        taskIds.forEach { id ->
-            val index = rows.indexOfFirst { it.id == id }
+    override fun markSynced(rows: List<TaskEntity>, href: String?, eTag: String?) {
+        rows.forEach { synced ->
+            val index = this.rows.indexOfFirst { it.id == synced.id }
             if (index >= 0) {
-                rows[index] = rows[index].copy(href = href, etag = eTag, isDirty = false)
+                val current = this.rows[index]
+                this.rows[index] = current.copy(href = href, etag = eTag, isDirty = current.lastModified != synced.lastModified)
             }
         }
     }

fastlane/metadata/android/ar/changelogs/1030199.txt

@@ -0,0 +1,6 @@
+### تغييرات
+- إنجاز مهمة متكررة ينقلها إلى موعدها التالي ويحتفظ بنسخة منجزة، فتراها التطبيقات الأخرى وواجهات الويب للخوادم منجزة.
+
+### إصلاحات
+- مع tasks.org تحتفظ المهام المتكررة بقاعدتها: تعديلها لم يعد يحذفها وإنجازها لم يعد ينهي السلسلة كلها.
+- مع tasks.org وOpenTasks يعرض المحرر التكرار ويُحفظ ترتيب القوائم.

fastlane/metadata/android/cs-CZ/changelogs/1030199.txt

@@ -0,0 +1,6 @@
+### Změny
+- Odškrtnutí opakovaného úkolu jej posune na další termín a ponechá dokončenou kopii, takže jiné aplikace a webová rozhraní serverů jej vidí jako hotový.
+
+### Opravy
+- S tasks.org si opakované úkoly drží své pravidlo: úprava ho už neodstraní a odškrtnutí už neukončí celou sérii.
+- S tasks.org a OpenTasks editor ukazuje opakování a pořadí seznamů zůstává zachováno.

fastlane/metadata/android/de-DE/changelogs/1030199.txt

@@ -0,0 +1,6 @@
+### Geändert
+- Eine wiederkehrende Aufgabe abzuhaken rückt sie zum nächsten Termin vor und behält eine erledigte Kopie, sodass andere Apps und Server-Weboberflächen sie als erledigt sehen.
+
+### Behoben
+- Mit tasks.org behalten wiederkehrende Aufgaben ihre Regel: Bearbeiten entfernt sie nicht mehr, Abhaken beendet nicht mehr die ganze Serie.
+- Mit tasks.org und OpenTasks zeigt der Editor die Wiederholung, und die Reihenfolge der Listen bleibt erhalten.

fastlane/metadata/android/en-GB/changelogs/1030199.txt

@@ -0,0 +1,6 @@
+### Changed
+- Ticking off a repeating task moves it on to its next date and keeps a completed copy, so other apps and server web views see it as done.
+
+### Fixed
+- With tasks.org, repeating tasks keep their rule: editing one no longer removes it and ticking one off no longer ends the series.
+- With tasks.org and OpenTasks, the editor shows the repeat rule and your list order is kept.

fastlane/metadata/android/en-US/changelogs/1030199.txt

@@ -0,0 +1,6 @@
+### Changed
+- Ticking off a repeating task moves it on to its next date and keeps a completed copy, so other apps and server web views see it as done.
+
+### Fixed
+- With tasks.org, repeating tasks keep their rule: editing one no longer removes it and ticking one off no longer ends the series.
+- With tasks.org and OpenTasks, the editor shows the repeat rule and your list order is kept.

fastlane/metadata/android/fr-FR/changelogs/1030199.txt

@@ -0,0 +1,6 @@
+### Modifications
+- Cocher une tâche récurrente la passe à sa prochaine date et garde une copie terminée : les autres applis et les interfaces web des serveurs la voient comme faite.
+
+### Corrections
+- Avec tasks.org, les tâches récurrentes gardent leur règle : la modifier ne la supprime plus, la cocher ne termine plus toute la série.
+- Avec tasks.org et OpenTasks, l’éditeur affiche la répétition et l’ordre des listes est conservé.

fastlane/metadata/android/pt-BR/changelogs/1030199.txt

@@ -0,0 +1,6 @@
+### Alterações
+- Concluir uma tarefa recorrente a leva para a próxima data e mantém uma cópia concluída, para que outros apps e a web do servidor a vejam como feita.
+
+### Correções
+- Com o tasks.org, tarefas recorrentes mantêm a regra: editar não a remove mais e concluir não encerra mais a série inteira.
+- Com tasks.org e OpenTasks, o editor mostra a repetição e a ordem das listas é mantida.

fastlane/metadata/android/ru-RU/changelogs/1030199.txt

@@ -0,0 +1,6 @@
+### Изменения
+- Отметка повторяющейся задачи переносит её на следующую дату и сохраняет выполненную копию, так что другие приложения и веб-интерфейсы серверов видят её выполненной.
+
+### Исправления
+- С tasks.org повторяющиеся задачи сохраняют правило: правка больше его не удаляет, а отметка не завершает всю серию.
+- С tasks.org и OpenTasks редактор показывает повторение, а порядок списков сохраняется.