fix(calendars): keep an event's own calendar when it is switched off

Excluding switched-off calendars from the event form's picker is right for
*targets*, but it also dropped the calendar an event already lives in. Editing
an event of a writable calendar switched off on this device (from a widget, a
deep link, another app's ACTION_EDIT) rendered the calendar row as the red "no
calendar" error, with the picker still enabled — so any pick turned the save
into a calendar move nobody asked for. The event's own calendar is added back
whenever it isn't among the targets, the way the managed special-dates case
already did; a calendar the app may not write to is still no target.

Settings → Notifications had missed the same predicate swap: it kept offering
per-calendar reminder overrides for switched-off calendars, where the provider
schedules no alarms and the setting could never fire.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/ui/edit/EventEditViewModel.kt

@@ -184,6 +184,9 @@ class EventEditViewModel @Inject constructor(
      * [state]). Managed special-dates calendars are excluded too: their events
      * are owned by the contact sync, which would delete any user event created
      * there.
+     *
+     * This is the list of *targets*. An event already living in an excluded
+     * calendar keeps it — [state] adds it back to the picker.
      */
     private val writableCalendars: Flow<List<CalendarSource>> = allCalendars.map { calendars ->
         calendars.filter { it.canModifyContents && it.isVisibleInSystem && !it.isManaged }
@@ -232,11 +235,16 @@ class EventEditViewModel @Inject constructor(
         // off the calendar's durable marker, not a stored id, so it holds after a
         // backup restore too.
         val isManaged = local.editTarget != null && resolvedCalendar?.isManaged == true
-        // The picker offers writable calendars only; when editing a managed event
-        // its own (excluded) calendar is added back so the row still names it.
+        // The picker offers writable calendars only; the event's own calendar is
+        // added back whenever it isn't among them — a managed special-dates one,
+        // or one switched off on this device — so the row keeps naming it instead
+        // of reading as the "no calendar" error, and saving can leave the event
+        // where it is. A calendar the app may not write to is still no target.
+        val ownCalendar = resolvedCalendar?.takeIf { own ->
+            own.canModifyContents && external.writable.none { it.id == own.id }
+        }
         val pickerCalendars =
-            if (isManaged && resolvedCalendar != null) external.writable + resolvedCalendar
-            else external.writable
+            if (ownCalendar != null) external.writable + ownCalendar else external.writable
         // An all-day event is date-anchored, so a zone is meaningless on it —
         // the field is withheld from both lists rather than shown as a no-op.
         val offerableFields = EventFormField.entries.toSet() -

app/src/main/java/de/jeanlucmakiola/calendula/ui/settings/SettingsViewModel.kt

@@ -74,9 +74,14 @@ class SettingsViewModel @Inject constructor(
 
     private val dynamicColorAvailable = Build.VERSION.SDK_INT >= Build.VERSION_CODES.S
 
-    /** Writable calendars — the only ones that take a per-calendar reminder override. */
+    /**
+     * Writable calendars that are switched on — the only ones that take a
+     * per-calendar reminder override. A calendar switched off in Settings →
+     * Calendars is `VISIBLE = 0`, so the provider schedules no alarms for it and
+     * a default reminder configured there could never fire (#75).
+     */
     private val writableCalendars: Flow<List<CalendarSource>> = repository.calendars()
-        .map { calendars -> calendars.filter { it.canModifyContents } }
+        .map { calendars -> calendars.filter { it.canModifyContents && it.isVisibleInSystem } }
         .catch { emit(emptyList()) }
 
     val state: StateFlow<SettingsUiState> =

app/src/test/java/de/jeanlucmakiola/calendula/ui/edit/EventEditViewModelTest.kt

@@ -108,6 +108,27 @@ class EventEditViewModelTest {
         job.cancel()
     }
 
+    @Test
+    fun `editing an event in a switched-off calendar keeps it in the picker`(
+        @TempDir tempDir: Path,
+    ) = runTest(dispatcher) {
+        // Otherwise the calendar row renders as the "no calendar" error and any
+        // pick routes the save through a move the user never asked for.
+        val fake = FakeCalendarDataSource().apply {
+            calendarsResult = listOf(cal(1L), cal(2L, visible = false))
+            eventDetailResult = { detail(calendarId = 2L) }
+        }
+        val vm = viewModel(tempDir, fake)
+        val job = activate(vm)
+
+        vm.openForEdit(eventId = 42L, beginMillis = beginMillis, endMillis = endMillis)
+        advanceUntilIdle()
+
+        assertThat(vm.state.value?.calendars?.map { it.id }).containsExactly(1L, 2L)
+        assertThat(vm.state.value?.form?.calendarId).isEqualTo(2L)
+        job.cancel()
+    }
+
     @Test
     fun `changing the calendar routes the save through a move, not an update`(
         @TempDir tempDir: Path,