fix(core-time): render all-day values in UTC, not the device zone

All-day items are date-only in iCalendar and are stored as UTC midnight of
their calendar date — the convention the tasks and calendar providers read
back (OpenTasks resolves them through DateTime.toAllDay(), which discards the
time and resolves the remaining date against UTC).

formatDate/formatDateTime/formatDateTimeCompact resolved every instant against
ZoneId.systemDefault(), so an all-day value rendered as the *previous* day for
anyone west of UTC: a task stored as 2026-07-20T00:00Z showed as 19 Jul in New
York. East of UTC it happened to land on the right day, which is why this went
unnoticed.

formatDate now takes an allDay flag (defaulting to false, so existing call
sites are unaffected) and all-day values resolve against UTC.

Callers must pass allDay at the call site — agendula and calendula both have
sites that need updating.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -6,10 +6,24 @@ All notable changes to floret-kit are documented here. The format follows
 floret-kit is consumed **from source** — each app embeds it as a git submodule
 and builds it through a Gradle composite build, pinning a specific commit. There
 are no binary releases; "version" tracks the shared `version` in the root build
-(currently `0.2.0`). Entries are grouped by module.
+(currently `0.2.1`). Entries are grouped by module.
 
 ## [Unreleased]
 
+## [0.2.1] - 2026-07-17
+
+### Added
+
+- **`components`** — `CollapsingScaffold` takes a `scrollable` flag (default
+  `true`, so every existing caller is unchanged). Set it to `false` when the
+  content brings its own scrolling container: the scaffold's own
+  `verticalScroll` composes each row up front and can't host a `LazyColumn`
+  (nesting a same-axis scrollable throws), which ruled out any surface with more
+  options than fit an eager pass. Opting out hands the content a plain column and
+  leaves the insets, IME padding and top-bar collapse as they were.
+  `FullScreenPicker` forwards the flag. Consumed by Calendula's ~600-entry
+  time-zone picker from 2.16.0.
+
 ## [0.2.0] - 2026-07-17
 
 ### Added

build.gradle.kts

@@ -10,5 +10,5 @@ plugins {
 
 subprojects {
     group = "de.jeanlucmakiola.floret"
-    version = "0.2.0"
+    version = "0.2.1"
 }

components/src/main/kotlin/de/jeanlucmakiola/floret/components/CollapsingScaffold.kt

@@ -46,6 +46,16 @@ import de.jeanlucmakiola.floret.identity.predictiveBack
  * unchanged: [actions] adds trailing app-bar items; [snackbarHost] hosts
  * transient messages; [predictiveBack] (off by default) wires the gesture
  * preview to [onBack] for full-screen surfaces that want it.
+ *
+ * Set [scrollable] to false when [content] brings its own scrolling container.
+ * The default column scrolls as a whole, which composes every row up front —
+ * fine for the short lists these surfaces normally hold, but a list long enough
+ * to need a [androidx.compose.foundation.lazy.LazyColumn] can neither nest one
+ * inside it (same-axis scrolling throws) nor afford the eager pass. Opting out
+ * hands [content] a plain, unscrolled column to fill instead; the insets, IME
+ * padding and bar behaviour are unchanged, so a lazy list simply takes over the
+ * scrolling and gets the top bar's collapse for free through the shared
+ * nested-scroll connection.
  */
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
@@ -55,6 +65,7 @@ fun CollapsingScaffold(
     modifier: Modifier = Modifier,
     largeTopBar: Boolean = true,
     predictiveBack: Boolean = false,
+    scrollable: Boolean = true,
     actions: @Composable RowScope.() -> Unit = {},
     snackbarHost: @Composable () -> Unit = {},
     content: @Composable ColumnScope.() -> Unit,
@@ -112,8 +123,19 @@ fun CollapsingScaffold(
                 .fillMaxSize()
                 .background(MaterialTheme.colorScheme.surface)
                 .imePadding()
+                .then(
+                    if (scrollable) {
+                        Modifier
                             .verticalScroll(rememberScrollState())
-                .padding(top = 8.dp, bottom = 24.dp),
+                            .padding(top = 8.dp, bottom = 24.dp)
+                    } else {
+                        // The content scrolls itself, so it also owns the edge
+                        // spacing — a lazy list wants it as contentPadding, or
+                        // the first and last rows clip instead of scrolling
+                        // under the bars.
+                        Modifier
+                    },
+                ),
             content = content,
         )
     }

components/src/main/kotlin/de/jeanlucmakiola/floret/components/OptionPicker.kt

@@ -20,12 +20,17 @@ import androidx.compose.ui.window.DialogWindowProvider
  * width without a tall header — it's a short selection list, so the large
  * collapsing header would only be empty space to scroll past. [content] places
  * the connected grouped rows; selecting one calls [onDismiss].
+ *
+ * [scrollable] forwards to [CollapsingScaffold]: leave it on unless [content]
+ * scrolls itself, which a picker only needs for an option list long enough to
+ * warrant a lazy container.
  */
 @Composable
 fun FullScreenPicker(
     title: String,
     onDismiss: () -> Unit,
     predictiveBack: Boolean = false,
+    scrollable: Boolean = true,
     actions: @Composable RowScope.() -> Unit = {},
     content: @Composable ColumnScope.() -> Unit,
 ) {
@@ -50,6 +55,7 @@ fun FullScreenPicker(
             onBack = onDismiss,
             largeTopBar = false,
             predictiveBack = predictiveBack,
+            scrollable = scrollable,
             actions = actions,
             content = content,
         )
@@ -96,8 +102,18 @@ fun <T> OptionPicker(
     }
 }
 
+/**
+ * The trailing check that marks the chosen row in a full-screen picker — the
+ * family's single selection affordance, so every picker (option, reminder,
+ * agenda range, timezone, calendar, recurrence) marks its selection the same
+ * way. Pair it with `GroupedRow(selected = true)`: the tonal highlight carries
+ * the state at a glance, the check confirms it unambiguously.
+ *
+ * Purely decorative — the row itself owns the semantics, so it takes no
+ * content description.
+ */
 @Composable
-private fun SelectedCheck() {
+fun SelectedCheck() {
     Icon(
         imageVector = Icons.Rounded.Check,
         contentDescription = null,

core-time/src/main/kotlin/de/jeanlucmakiola/floret/time/DateTimeFormat.kt

@@ -3,6 +3,7 @@ package de.jeanlucmakiola.floret.time
 import java.time.LocalDate
 import java.time.LocalTime
 import java.time.ZoneId
+import java.time.ZoneOffset
 import java.time.chrono.IsoChronology
 import java.time.format.DateTimeFormatter
 import java.time.format.DateTimeFormatterBuilder
@@ -22,15 +23,28 @@ private fun Instant.atSystemZone() =
     java.time.Instant.ofEpochMilli(toEpochMilliseconds())
         .atZone(ZoneId.systemDefault())
 
+/**
+ * The zone an instant should be read in to recover the date it denotes.
+ *
+ * All-day items are date-only in iCalendar and are stored as **UTC midnight** of
+ * their calendar date (the convention the tasks/calendar providers read back).
+ * Rendering one in the device zone shows the previous day for anyone west of
+ * UTC — so all-day values resolve against UTC, timed values against the device.
+ */
+private fun Instant.atDisplayZone(allDay: Boolean) =
+    if (allDay) java.time.Instant.ofEpochMilli(toEpochMilliseconds()).atZone(ZoneOffset.UTC)
+    else atSystemZone()
+
 /** Medium localized date, e.g. "18 Jun 2026". */
-fun Instant.formatDate(): String = atSystemZone().format(dateFormatter)
+fun Instant.formatDate(allDay: Boolean = false): String =
+    atDisplayZone(allDay).format(dateFormatter)
 
 /** Short localized time, e.g. "14:30". */
 fun Instant.formatTime(): String = atSystemZone().format(timeFormatter)
 
 /** Date alone for all-day items, otherwise date + time. */
 fun Instant.formatDateTime(allDay: Boolean): String =
-    if (allDay) formatDate() else "${formatDate()} · ${formatTime()}"
+    if (allDay) formatDate(allDay = true) else "${formatDate()} · ${formatTime()}"
 
 // The locale's medium date pattern with the year token stripped, e.g. "20 Jun" /
 // "Jun 20" — derived from the pattern so day/month order still follows the locale.
@@ -49,10 +63,10 @@ private val dateNoYearFormatter: DateTimeFormatter = run {
  * items, or a midnight time). Examples: "20 Jun", "20 Jun · 14:30", "20 Jun 2027".
  */
 fun Instant.formatDateTimeCompact(allDay: Boolean): String {
-    val zdt = atSystemZone()
+    val zdt = atDisplayZone(allDay)
     val datePart =
         if (zdt.year == LocalDate.now(ZoneId.systemDefault()).year) zdt.format(dateNoYearFormatter)
-        else formatDate()
+        else formatDate(allDay)
     return if (allDay || zdt.toLocalTime() == LocalTime.MIDNIGHT) datePart
     else "$datePart · ${formatTime()}"
 }

core-time/src/test/kotlin/de/jeanlucmakiola/floret/time/DateTimeFormatTest.kt

@@ -0,0 +1,76 @@
+package de.jeanlucmakiola.floret.time
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.AfterEach
+import org.junit.jupiter.api.Test
+import java.time.LocalDate
+import java.time.ZoneOffset
+import java.time.format.DateTimeFormatter
+import java.time.format.FormatStyle
+import java.util.TimeZone
+import kotlin.time.Instant
+
+class DateTimeFormatTest {
+
+    private val originalZone: TimeZone = TimeZone.getDefault()
+
+    @AfterEach
+    fun restoreZone() = TimeZone.setDefault(originalZone)
+
+    private val july20: LocalDate = LocalDate.of(2026, 7, 20)
+
+    /** The all-day storage form: UTC midnight of the calendar date. */
+    private val allDayJuly20: Instant = Instant.fromEpochMilliseconds(
+        july20.atStartOfDay(ZoneOffset.UTC).toInstant().toEpochMilli(),
+    )
+
+    /** What the locale renders for a given date — so the test doesn't pin a format. */
+    private fun medium(date: LocalDate): String =
+        date.format(DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM))
+
+    @Test
+    fun `an all-day date reads the same whatever the device zone`() {
+        TimeZone.setDefault(TimeZone.getTimeZone("America/New_York")) // UTC-4 in July
+        assertThat(allDayJuly20.formatDate(allDay = true)).isEqualTo(medium(july20))
+
+        TimeZone.setDefault(TimeZone.getTimeZone("Europe/Berlin")) // UTC+2 in July
+        assertThat(allDayJuly20.formatDate(allDay = true)).isEqualTo(medium(july20))
+
+        TimeZone.setDefault(TimeZone.getTimeZone("Pacific/Auckland")) // UTC+12
+        assertThat(allDayJuly20.formatDate(allDay = true)).isEqualTo(medium(july20))
+    }
+
+    @Test
+    fun `reading an all-day value as timed slips a day west of UTC`() {
+        // Guards the regression directly: this is what the all-day flag prevents.
+        TimeZone.setDefault(TimeZone.getTimeZone("America/New_York"))
+        assertThat(allDayJuly20.formatDate()).isEqualTo(medium(july20.minusDays(1)))
+        assertThat(allDayJuly20.formatDate(allDay = true)).isEqualTo(medium(july20))
+    }
+
+    @Test
+    fun `timed values still render in the device zone`() {
+        // 2026-07-20T23:30Z — the next day already, in Auckland.
+        val lateUtc = Instant.fromEpochMilliseconds(
+            july20.atTime(23, 30).atZone(ZoneOffset.UTC).toInstant().toEpochMilli(),
+        )
+        TimeZone.setDefault(TimeZone.getTimeZone("Pacific/Auckland"))
+        assertThat(lateUtc.formatDate()).isEqualTo(medium(july20.plusDays(1)))
+    }
+
+    @Test
+    fun `formatDateTime drops the time for all-day and keeps it otherwise`() {
+        TimeZone.setDefault(TimeZone.getTimeZone("Europe/Berlin"))
+        assertThat(allDayJuly20.formatDateTime(allDay = true)).isEqualTo(medium(july20))
+        assertThat(allDayJuly20.formatDateTime(allDay = false)).contains("·")
+    }
+
+    @Test
+    fun `compact form is zone-correct for all-day too`() {
+        TimeZone.setDefault(TimeZone.getTimeZone("America/New_York"))
+        val compact = allDayJuly20.formatDateTimeCompact(allDay = true)
+        // No time part, and the day must not have slipped to the 19th.
+        assertThat(compact).doesNotContain("·")
+        assertThat(compact).contains("20")
+    }
+}