Widgets: close the rest of the rollover gaps (#228)

Review follow-up on the midnight alarm.

The month widget could still show the wrong month after the rollover, and by way
of the exact workaround #228's reporter described. ShiftMonthAction stored an
absolute index on every tap, so paging forward and back — which looks like a
no-op and was how people forced a redraw — silently pinned the widget to the
month that was current at the time. It kept redrawing correctly and kept drawing
August into September, today's circle nowhere on the grid. Landing back on the
current month now clears the key instead of writing it, so the widget goes back
to following the date. Paging somewhere else and staying there is left alone;
that one is a choice, and the today button undoes it.

The alarm also had no way back once something dropped it without telling us — a
force-stop, a battery-restricted transition, an OEM freeze — short of the user
opening the app. onUpdate is the one wake-up the system still owns through
updatePeriodMillis, so both receivers re-arm from it. That doubles as a window
narrower: setAndAllowWhileIdle's delivery slack scales with how far out the alarm
was set, so re-arming every half hour keeps midnight's window at minutes rather
than hours.

Boot and package-replace now re-arm and stop there. The host sends
APPWIDGET_UPDATE after both, so the redraw was a second pair of wide provider
reads and RemoteViews serialisations in a cold process, exactly when the device
is busiest.

Smaller things: the unreachable lookahead fallback armed 5 seconds out, which
would have been a wake-loop rather than the late redraw the comment claimed —
an hour now. The action guard's comment was copied from ReminderScheduleReceiver
along with its claim that the broadcasts are protected; PROVIDER_CHANGED is not,
and the real reason the guard is worth having is that the receiver must stay
exported. Application start does its sync off the main thread now.

Tests moved onto frozen historical transitions — Berlin 2024 both ways, Havana
2018 — since the 2026 dates they used depend on DST rules that can still change
under a tzdata bump. The Berlin "fall back" case never repeated midnight, so it
now says what it actually checks (a 25-hour day must not overshoot) and a real
repeated-midnight zone, pre-2019 Sao Paulo, pins the known limitation instead of
implying it is handled. Dropped the two assertions that restated the code, and
added the one that was missing: that the receiver's action guard admits the
action the alarm is sent with, which is the single point where the whole thing
would fail silently.

CHANGELOG.md

@@ -7,6 +7,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- **Home-screen widgets now turn the page at midnight.** Both the month and the
+  agenda widget kept highlighting yesterday as "today" — and the agenda kept
+  greying out the wrong events as already past — until you paged the month back
+  and forth or removed and re-added the widget. Calendula now wakes itself at the
+  day boundary and redraws, and re-arms after a reboot, a clock change or a
+  flight into another timezone. Paging the month widget forward and back also
+  stops quietly pinning it to that month, so it follows the date again instead of
+  being stranded on the month you happened to be looking at ([#228]).
+
 ## [2.19.3] — 2026-08-22
 
 ### Added
@@ -1471,3 +1481,4 @@ automatically, with zero telemetry and no internet permission.
 [#192]: https://codeberg.org/jlmakiola/calendula/issues/192
 [#196]: https://codeberg.org/jlmakiola/calendula/issues/196
 [#214]: https://codeberg.org/jlmakiola/calendula/issues/214
+[#228]: https://codeberg.org/jlmakiola/calendula/issues/228

app/proguard-rules.pro

@@ -50,3 +50,12 @@
 # the real names also survives app updates, which would otherwise renumber the
 # obfuscated name and orphan the stored mapping.
 -keep class * extends androidx.glance.appwidget.GlanceAppWidget
+
+# Belt and braces one level up: MonthWidgetReceiver and AgendaWidgetReceiver are
+# nearly as alike (same supertype, same overrides, only a differing property
+# initializer), and Glance's provider map is keyed off the receiver component
+# too. AGP's manifest-derived keep rules already cover them, and the rule above
+# keeps the two widgets distinct enough that the receivers' constructors differ —
+# so this is redundant today. It is here because #89 cost a release to diagnose
+# and the guarantee should not rest on a component staying in the manifest.
+-keep class * extends androidx.glance.appwidget.GlanceAppWidgetReceiver

app/src/main/AndroidManifest.xml

@@ -331,8 +331,13 @@
 
         <!-- Keeps both widgets fresh: the calendar provider broadcasts
              PROVIDER_CHANGED on any data change (our writes and external sync),
-             and the system broadcasts the date/time ones at midnight / clock
-             changes so "today" highlighting rolls over. -->
+             and the day boundary arrives as the app's own ROLLOVER alarm (#228),
+             delivered by an explicit PendingIntent so it needs no filter here.
+             DATE_CHANGED is kept as a free extra only — it is not an exempted
+             implicit broadcast, so a manifest-declared receiver is not given it
+             on Android 8+. TIME_SET / TIMEZONE_CHANGED move the day boundary,
+             and boot / package-replace wipe the alarm, so all four re-arm it.
+             Exported: the system broadcasts arrive from outside the app. -->
         <receiver
             android:name=".widget.WidgetUpdateReceiver"
             android:exported="true">
@@ -346,6 +351,8 @@
                 <action android:name="android.intent.action.DATE_CHANGED" />
                 <action android:name="android.intent.action.TIME_SET" />
                 <action android:name="android.intent.action.TIMEZONE_CHANGED" />
+                <action android:name="android.intent.action.BOOT_COMPLETED" />
+                <action android:name="android.intent.action.MY_PACKAGE_REPLACED" />
             </intent-filter>
         </receiver>
 

app/src/main/java/de/jeanlucmakiola/calendula/CalendulaApp.kt

@@ -10,6 +10,7 @@ import de.jeanlucmakiola.calendula.data.contacts.SpecialDatesScheduler
 import de.jeanlucmakiola.calendula.data.contacts.SpecialDatesSyncWorker
 import de.jeanlucmakiola.calendula.data.reminders.ReminderMaintenanceScheduler
 import de.jeanlucmakiola.calendula.data.reminders.ReminderMaintenanceWorker
+import de.jeanlucmakiola.calendula.widget.WidgetRolloverScheduler
 import de.jeanlucmakiola.floret.crash.CrashConfig
 import de.jeanlucmakiola.floret.crash.CrashReporter
 import kotlinx.coroutines.CoroutineScope
@@ -44,6 +45,21 @@ class CalendulaApp : Application() {
         reconcileSpecialDates()
         reconcileCalendarVisibility()
         startReminderDelivery()
+        reconcileWidgetRollover()
+    }
+
+    /**
+     * Re-arm the widgets' midnight rollover from whatever is actually placed
+     * (#228). Idempotent, and it covers the cases no broadcast reaches — an
+     * install upgrading into the fix, or an alarm dropped by a force-stop, is
+     * armed again the next time the app is opened. Off the main thread because
+     * it makes a handful of binder calls and every process start runs it,
+     * including ones a worker or a receiver triggered.
+     */
+    private fun reconcileWidgetRollover() {
+        CoroutineScope(SupervisorJob() + Dispatchers.Default).launch {
+            WidgetRolloverScheduler.sync(this@CalendulaApp)
+        }
     }
 
     /**

app/src/main/java/de/jeanlucmakiola/calendula/widget/WidgetRolloverScheduler.kt

@@ -0,0 +1,122 @@
+package de.jeanlucmakiola.calendula.widget
+
+import android.app.AlarmManager
+import android.app.PendingIntent
+import android.appwidget.AppWidgetManager
+import android.content.ComponentName
+import android.content.Context
+import android.content.Intent
+import androidx.core.content.getSystemService
+import de.jeanlucmakiola.calendula.widget.agenda.AgendaWidgetReceiver
+import de.jeanlucmakiola.calendula.widget.month.MonthWidgetReceiver
+import kotlinx.datetime.DateTimeUnit
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.atStartOfDayIn
+import kotlinx.datetime.plus
+import kotlinx.datetime.toLocalDateTime
+import kotlin.time.Clock
+import kotlin.time.Duration.Companion.hours
+import kotlin.time.Duration.Companion.seconds
+import kotlin.time.Instant
+
+/**
+ * Holds the app's own wake-up for the next local midnight, so the home-screen
+ * widgets roll "today" over on the day boundary (#228).
+ *
+ * The widgets used to lean on `ACTION_DATE_CHANGED`, but that broadcast is not
+ * on the implicit-broadcast exemption list, so a manifest-declared receiver has
+ * never been given it since Android 8 — leaving only `updatePeriodMillis`, which
+ * the system defers in doze and OEM skins throttle harder still. The result was
+ * yesterday staying highlighted (and the agenda's past-event dimming staying
+ * anchored to yesterday) until something else forced a redraw.
+ *
+ * Exactly one alarm exists at a time and every firing re-arms the next one, the
+ * same shape as [de.jeanlucmakiola.calendula.data.reminders.ReminderAlarmScheduler].
+ * It is deliberately **inexact**: `setAndAllowWhileIdle` needs no permission and
+ * survives doze (which plain `set` does not), and a rollover that lands a few
+ * minutes late is invisible on a sleeping screen. Exact alarms stay reserved for
+ * reminder snooze.
+ */
+object WidgetRolloverScheduler {
+
+    /**
+     * Fire just *after* midnight, never exactly on it. An alarm delivered a few
+     * milliseconds early would still read the old date and re-arm for an instant
+     * later; the offset makes "the day has changed" unambiguous.
+     */
+    internal val ROLLOVER_SLACK = 5.seconds
+
+    /**
+     * Arm the next rollover, or cancel a pending one when no widget is placed.
+     * Idempotent, so every trigger (boot, app start, widget added/removed, the
+     * rollover itself, a clock or timezone change) can just call it.
+     */
+    fun sync(context: Context) {
+        val appContext = context.applicationContext
+        val alarmManager = appContext.getSystemService<AlarmManager>() ?: return
+        val pendingIntent = rolloverPendingIntent(appContext)
+        if (!hasPlacedWidgets(appContext)) {
+            alarmManager.cancel(pendingIntent)
+            return
+        }
+        val triggerAt = nextRolloverAt(Clock.System.now(), TimeZone.currentSystemDefault())
+        alarmManager.setAndAllowWhileIdle(
+            AlarmManager.RTC_WAKEUP, triggerAt.toEpochMilliseconds(), pendingIntent,
+        )
+    }
+
+    /**
+     * The instant just after the next local midnight following [now] in [zone].
+     *
+     * Uses the *actual* start of the day rather than 00:00, so it stays correct
+     * where a DST jump means midnight never happens (Havana springs from 00:00 to
+     * 01:00) and where a whole local date is skipped by a date-line move (Apia
+     * had no 30 December 2011) — the loop then walks on to the next real day.
+     *
+     * The mirror case, a zone that rewinds *across* midnight so the day starts
+     * twice, resolves to the earlier start; the widget would then run an hour
+     * ahead of the clock. No entry in the current tz database does that (Brazil,
+     * which used to, dropped DST in 2019), and `updatePeriodMillis` covers it,
+     * so it is not worth carrying state to detect.
+     */
+    fun nextRolloverAt(now: Instant, zone: TimeZone): Instant {
+        val date = now.toLocalDateTime(zone).date
+        var days = 1
+        while (days <= MAX_LOOKAHEAD_DAYS) {
+            val candidate = date.plus(days, DateTimeUnit.DAY).atStartOfDayIn(zone) + ROLLOVER_SLACK
+            if (candidate > now) return candidate
+            days++
+        }
+        // Unreachable for any zone in the tz database. Deliberately an hour and
+        // not the slack: a 5-second retry would just hit this branch again and
+        // wake the device in a loop.
+        return now + 1.hours
+    }
+
+    private fun hasPlacedWidgets(context: Context): Boolean {
+        val manager = AppWidgetManager.getInstance(context) ?: return false
+        return PROVIDERS.any {
+            manager.getAppWidgetIds(ComponentName(context, it)).isNotEmpty()
+        }
+    }
+
+    private fun rolloverPendingIntent(context: Context): PendingIntent =
+        PendingIntent.getBroadcast(
+            context,
+            ROLLOVER_REQUEST_CODE,
+            Intent(context, WidgetUpdateReceiver::class.java)
+                .setAction(WidgetUpdateReceiver.ACTION_ROLLOVER),
+            PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE,
+        )
+
+    private val PROVIDERS = listOf(
+        MonthWidgetReceiver::class.java,
+        AgendaWidgetReceiver::class.java,
+    )
+
+    /** Fixed: there is only ever one rollover alarm, and re-arming must replace it. */
+    private const val ROLLOVER_REQUEST_CODE = 0x0DA1
+
+    /** A gap of more than a couple of days does not exist in any tz database entry. */
+    private const val MAX_LOOKAHEAD_DAYS = 3
+}

app/src/main/java/de/jeanlucmakiola/calendula/widget/WidgetUpdateReceiver.kt

@@ -12,19 +12,47 @@ import kotlinx.coroutines.SupervisorJob
 import kotlinx.coroutines.launch
 
 /**
- * Redraws both home-screen widgets when their data goes stale. Triggered by:
+ * Redraws both home-screen widgets when their data goes stale, and keeps the
+ * midnight rollover alarm armed. Triggered by:
  *  - `PROVIDER_CHANGED` from the calendar provider — fires on any data change,
  *    so it covers both the app's own writes and external sync.
- *  - `DATE_CHANGED` / `TIME_SET` / `TIMEZONE_CHANGED` — so "today" highlighting
- *    and the upcoming window roll over at midnight / on a clock change.
+ *  - [ACTION_ROLLOVER], the app's own alarm from [WidgetRolloverScheduler] —
+ *    the day boundary, so "today" highlighting and the agenda's past-event
+ *    dimming move on (#228).
+ *  - `TIME_SET` / `TIMEZONE_CHANGED` — a clock or zone change moves the day
+ *    boundary relative to the armed alarm, so both redraw *and* re-arm.
+ *  - `BOOT_COMPLETED` / `MY_PACKAGE_REPLACED` — both wipe pending alarms. The
+ *    package-replaced one is also what arms existing installs that upgrade into
+ *    the fix without re-adding their widget.
  *
- * Both widgets also carry an `updatePeriodMillis` backstop in their provider
- * XML, and the month widget's refresh button forces an immediate redraw.
+ * `DATE_CHANGED` is still in the manifest filter as a free extra, but nothing
+ * depends on it: it is not an exempted implicit broadcast, so a manifest-declared
+ * receiver has not actually been given it since Android 8. The widgets also carry
+ * an `updatePeriodMillis` backstop in their provider XML, and the month widget's
+ * refresh button forces an immediate redraw.
+ *
+ * Exported for the system broadcasts; an extra redraw triggered by another app
+ * is harmless.
  */
 class WidgetUpdateReceiver : BroadcastReceiver() {
     override fun onReceive(context: Context, intent: Intent) {
-        val pending = goAsync()
+        // The receiver has to stay exported for the system broadcasts, so an
+        // explicit intent can reach it with anything in it. Nothing here reads
+        // the intent's data and nothing crosses a trust boundary, but narrowing
+        // to the actions we actually asked for keeps a stray broadcast from
+        // costing two wide provider reads.
+        if (intent.action !in HANDLED_ACTIONS) return
         val appContext = context.applicationContext
+        // Re-arm first: whatever happens to the redraw, the next day boundary is
+        // covered. Boot and package-replace dropped the alarm outright; a
+        // rollover just consumed it; a clock change invalidated it.
+        WidgetRolloverScheduler.sync(appContext)
+        // Boot and package-replace only cost us the alarm. The host sends
+        // APPWIDGET_UPDATE after both anyway, so redrawing here would just repeat
+        // two wide provider reads and two RemoteViews serialisations in a cold
+        // process, at the moment the device is most contended.
+        if (intent.action in REARM_ONLY_ACTIONS) return
+        val pending = goAsync()
         // Calendar data may have changed (sync / our own write) — drop the cached
         // month window so the widgets reload fresh. Month paging does NOT call
         // this, so arrow taps stay instant.
@@ -38,4 +66,23 @@ class WidgetUpdateReceiver : BroadcastReceiver() {
             }
         }
     }
+
+    companion object {
+        /** The app's own midnight wake-up; see [WidgetRolloverScheduler]. */
+        const val ACTION_ROLLOVER = "de.jeanlucmakiola.calendula.widget.ROLLOVER"
+
+        /** Both wipe pending alarms, and the host redraws the widgets itself after them. */
+        private val REARM_ONLY_ACTIONS = setOf(
+            Intent.ACTION_BOOT_COMPLETED,
+            Intent.ACTION_MY_PACKAGE_REPLACED,
+        )
+
+        internal val HANDLED_ACTIONS = REARM_ONLY_ACTIONS + setOf(
+            ACTION_ROLLOVER,
+            Intent.ACTION_PROVIDER_CHANGED,
+            Intent.ACTION_DATE_CHANGED,
+            Intent.ACTION_TIME_CHANGED,
+            Intent.ACTION_TIMEZONE_CHANGED,
+        )
+    }
 }

app/src/main/java/de/jeanlucmakiola/calendula/widget/agenda/AgendaWidgetReceiver.kt

@@ -1,7 +1,10 @@
 package de.jeanlucmakiola.calendula.widget.agenda
 
+import android.appwidget.AppWidgetManager
+import android.content.Context
 import androidx.glance.appwidget.GlanceAppWidget
 import androidx.glance.appwidget.GlanceAppWidgetReceiver
+import de.jeanlucmakiola.calendula.widget.WidgetRolloverScheduler
 
 /**
  * Host-facing receiver for the agenda widget. Declared in the manifest with the
@@ -10,4 +13,33 @@ import androidx.glance.appwidget.GlanceAppWidgetReceiver
  */
 class AgendaWidgetReceiver : GlanceAppWidgetReceiver() {
     override val glanceAppWidget: GlanceAppWidget = AgendaWidget()
+
+    /** First agenda widget placed — start rolling "today" over at midnight (#228). */
+    override fun onEnabled(context: Context) {
+        super.onEnabled(context)
+        WidgetRolloverScheduler.sync(context)
+    }
+
+    /**
+     * Last agenda widget removed. [WidgetRolloverScheduler.sync] only cancels the
+     * alarm if no month widget is left either, so removing one kind never stops
+     * the other from rolling over.
+     */
+    override fun onDisabled(context: Context) {
+        super.onDisabled(context)
+        WidgetRolloverScheduler.sync(context)
+    }
+
+    /**
+     * Self-heal on the system's own `updatePeriodMillis` wake-up — see
+     * [de.jeanlucmakiola.calendula.widget.month.MonthWidgetReceiver.onUpdate].
+     */
+    override fun onUpdate(
+        context: Context,
+        appWidgetManager: AppWidgetManager,
+        appWidgetIds: IntArray,
+    ) {
+        super.onUpdate(context, appWidgetManager, appWidgetIds)
+        WidgetRolloverScheduler.sync(context)
+    }
 }

app/src/main/java/de/jeanlucmakiola/calendula/widget/month/MonthWidget.kt

@@ -152,7 +152,19 @@ class ShiftMonthAction : ActionCallback {
         val delta = parameters[deltaKey] ?: 0
         updateAppWidgetState(context, glanceId) { prefs ->
             val cur = prefs[MONTH_INDEX_KEY] ?: currentMonthIndex(systemZone())
-            prefs[MONTH_INDEX_KEY] = cur + delta
+            val next = cur + delta
+            // Landing back on the current month clears the key rather than
+            // storing today's index, so the widget goes back to *following* the
+            // date instead of being pinned to the month that happened to be
+            // current when it was tapped. Paging forward and back is the very
+            // workaround #228's reporter used to force a redraw; storing the
+            // index there would have left them stuck on that month for good once
+            // it stopped being the current one.
+            if (next == currentMonthIndex(systemZone())) {
+                prefs.remove(MONTH_INDEX_KEY)
+            } else {
+                prefs[MONTH_INDEX_KEY] = next
+            }
         }
         MonthWidget().update(context.applicationContext, glanceId)
     }

app/src/main/java/de/jeanlucmakiola/calendula/widget/month/MonthWidgetReceiver.kt

@@ -1,7 +1,10 @@
 package de.jeanlucmakiola.calendula.widget.month
 
+import android.appwidget.AppWidgetManager
+import android.content.Context
 import androidx.glance.appwidget.GlanceAppWidget
 import androidx.glance.appwidget.GlanceAppWidgetReceiver
+import de.jeanlucmakiola.calendula.widget.WidgetRolloverScheduler
 
 /**
  * Host-facing receiver for the month widget. Declared in the manifest with the
@@ -9,4 +12,37 @@ import androidx.glance.appwidget.GlanceAppWidgetReceiver
  */
 class MonthWidgetReceiver : GlanceAppWidgetReceiver() {
     override val glanceAppWidget: GlanceAppWidget = MonthWidget()
+
+    /** First month widget placed — start rolling "today" over at midnight (#228). */
+    override fun onEnabled(context: Context) {
+        super.onEnabled(context)
+        WidgetRolloverScheduler.sync(context)
+    }
+
+    /**
+     * Last month widget removed. [WidgetRolloverScheduler.sync] only cancels the
+     * alarm if no agenda widget is left either, so removing one kind never stops
+     * the other from rolling over.
+     */
+    override fun onDisabled(context: Context) {
+        super.onDisabled(context)
+        WidgetRolloverScheduler.sync(context)
+    }
+
+    /**
+     * The `updatePeriodMillis` backstop is the one wake-up the *system* still
+     * owns, so it doubles as the rollover alarm's self-heal: anything that drops
+     * a pending alarm without a broadcast — a force-stop, a battery-restricted
+     * transition, an OEM freeze — is repaired here rather than waiting for the
+     * app to be opened. Re-arming closer to midnight also narrows the inexact
+     * alarm's delivery window, which scales with how far out it was set.
+     */
+    override fun onUpdate(
+        context: Context,
+        appWidgetManager: AppWidgetManager,
+        appWidgetIds: IntArray,
+    ) {
+        super.onUpdate(context, appWidgetManager, appWidgetIds)
+        WidgetRolloverScheduler.sync(context)
+    }
 }

app/src/test/java/de/jeanlucmakiola/calendula/widget/WidgetRolloverSchedulerTest.kt

@@ -0,0 +1,166 @@
+package de.jeanlucmakiola.calendula.widget
+
+import com.google.common.truth.Truth.assertThat
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.LocalDateTime
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.atStartOfDayIn
+import kotlinx.datetime.toInstant
+import kotlinx.datetime.toLocalDateTime
+import org.junit.jupiter.api.Test
+import kotlin.time.Duration
+import kotlin.time.Duration.Companion.hours
+import kotlin.time.Duration.Companion.minutes
+import kotlin.time.Instant
+
+/**
+ * The rollover alarm is what makes the widgets stop highlighting yesterday
+ * (#228), so the "when is the next local midnight" arithmetic is the one piece
+ * worth pinning down — especially where midnight is not 00:00.
+ */
+class WidgetRolloverSchedulerTest {
+
+    private val berlin = TimeZone.of("Europe/Berlin")
+
+    private fun at(local: String, zone: TimeZone): Instant =
+        LocalDateTime.parse(local).toInstant(zone)
+
+    private fun nextRollover(local: String, zone: TimeZone = berlin): Instant =
+        WidgetRolloverScheduler.nextRolloverAt(at(local, zone), zone)
+
+    // --- the ordinary day ----------------------------------------------------
+
+    @Test
+    fun `midday rolls over at the coming midnight`() {
+        val next = nextRollover("2026-08-27T12:00:00")
+        assertThat(next).isEqualTo(at("2026-08-28T00:00:05", berlin))
+    }
+
+    @Test
+    fun `a second before midnight still targets tonight, not tomorrow night`() {
+        val next = nextRollover("2026-08-27T23:59:59")
+        assertThat(next).isEqualTo(at("2026-08-28T00:00:05", berlin))
+    }
+
+    @Test
+    fun `at midnight exactly the target is the next day, never the current instant`() {
+        // The alarm has just fired and is re-arming: it must move a whole day on,
+        // otherwise the widget would wake itself in a tight loop.
+        val now = at("2026-08-28T00:00:00", berlin)
+        val next = WidgetRolloverScheduler.nextRolloverAt(now, berlin)
+        assertThat(next).isEqualTo(at("2026-08-29T00:00:05", berlin))
+        assertThat(next - now).isGreaterThan(Duration.ZERO)
+    }
+
+    @Test
+    fun `re-arming from the slack instant itself moves a full day on`() {
+        // What actually happens in practice: the receiver runs at midnight + slack.
+        val now = at("2026-08-28T00:00:05", berlin)
+        assertThat(WidgetRolloverScheduler.nextRolloverAt(now, berlin))
+            .isEqualTo(at("2026-08-29T00:00:05", berlin))
+    }
+
+    @Test
+    fun `the result is always in the future for every minute of a day`() {
+        val zone = berlin
+        // Spans Berlin's 2024 spring-forward, the case most likely to produce a
+        // target in the past and so an alarm that fires immediately, forever.
+        var probe = LocalDateTime.parse("2024-03-29T00:00:00").toInstant(zone)
+        val end = LocalDateTime.parse("2024-04-01T00:00:00").toInstant(zone)
+        while (probe < end) {
+            assertThat(WidgetRolloverScheduler.nextRolloverAt(probe, zone)).isGreaterThan(probe)
+            probe += 1.minutes
+        }
+    }
+
+    // --- daylight saving -----------------------------------------------------
+
+    @Test
+    fun `spring forward keeps the rollover one day away, not one hour short`() {
+        // Berlin skipped 02:00-03:00 on 31 March 2024, so that day was 23h long.
+        // A rollover computed as "now + 24h" would land at 01:00 on 1 April.
+        val now = at("2024-03-30T12:00:00", berlin)
+        val next = WidgetRolloverScheduler.nextRolloverAt(now, berlin)
+        assertThat(next).isEqualTo(at("2024-03-31T00:00:05", berlin))
+        assertThat(next - now).isLessThan(24.hours)
+    }
+
+    @Test
+    fun `fall back does not overshoot into the repeated hour`() {
+        // Berlin repeated 02:00-03:00 on 27 October 2024: midnight itself is
+        // unambiguous, but the day is 25h long, so "now + 24h" would land at
+        // 23:00 on the 26th and never roll the date over at all.
+        val now = at("2024-10-26T12:00:00", berlin)
+        val next = WidgetRolloverScheduler.nextRolloverAt(now, berlin)
+        assertThat(next).isEqualTo(at("2024-10-27T00:00:05", berlin))
+        assertThat(next.toLocalDateTime(berlin).date).isEqualTo(LocalDate.parse("2024-10-27"))
+    }
+
+    @Test
+    fun `a zone that repeats midnight takes the first start of day`() {
+        // Sao Paulo used to end DST by moving 00:00 back to 23:00, so the day
+        // began twice. Pinned deliberately: the widget then runs an hour ahead
+        // of the clock until the next redraw, which is the accepted trade
+        // (Brazil dropped DST in 2019, so no live zone does this).
+        val saoPaulo = TimeZone.of("America/Sao_Paulo")
+        val next = WidgetRolloverScheduler.nextRolloverAt(
+            at("2018-02-16T12:00:00", saoPaulo), saoPaulo,
+        )
+        val local = next.toLocalDateTime(saoPaulo)
+        assertThat(local.date).isEqualTo(LocalDate.parse("2018-02-17"))
+        assertThat(local.hour).isEqualTo(0)
+    }
+
+    @Test
+    fun `a zone where midnight does not exist rolls over at the real start of day`() {
+        // Cuba starts DST at 00:00, so 11 March 2018 began at 01:00 in Havana.
+        // Targeting a literal 00:00 there would arm an instant on the wrong day.
+        val havana = TimeZone.of("America/Havana")
+        val next = WidgetRolloverScheduler.nextRolloverAt(at("2018-03-10T12:00:00", havana), havana)
+        val local = next.toLocalDateTime(havana)
+        assertThat(local.date).isEqualTo(LocalDate.parse("2018-03-11"))
+        assertThat(local.hour).isEqualTo(1)
+        assertThat(local.minute).isEqualTo(0)
+        // And it is genuinely the first instant of that date, not a guess.
+        assertThat(next).isEqualTo(
+            LocalDate.parse("2018-03-11").atStartOfDayIn(havana) +
+                WidgetRolloverScheduler.ROLLOVER_SLACK,
+        )
+    }
+
+    // --- timezone changes ----------------------------------------------------
+
+    @Test
+    fun `the same instant rolls over at different times in different zones`() {
+        // Flying east and getting TIMEZONE_CHANGED must re-arm to the new local
+        // midnight — the arithmetic follows the zone, not a cached offset.
+        val instant = at("2026-08-27T12:00:00", berlin)
+        val tokyo = TimeZone.of("Asia/Tokyo")
+        val berlinNext = WidgetRolloverScheduler.nextRolloverAt(instant, berlin)
+        val tokyoNext = WidgetRolloverScheduler.nextRolloverAt(instant, tokyo)
+        assertThat(tokyoNext).isNotEqualTo(berlinNext)
+        assertThat(tokyoNext).isLessThan(berlinNext)
+        assertThat(tokyoNext.toLocalDateTime(tokyo).hour).isEqualTo(0)
+    }
+
+    @Test
+    fun `a half-hour offset zone still lands on its own midnight`() {
+        val kathmandu = TimeZone.of("Asia/Kathmandu")
+        val next = WidgetRolloverScheduler.nextRolloverAt(
+            at("2026-08-27T12:00:00", kathmandu), kathmandu,
+        )
+        val local = next.toLocalDateTime(kathmandu)
+        assertThat(local.date.toString()).isEqualTo("2026-08-28")
+        assertThat(local.hour).isEqualTo(0)
+    }
+
+    // --- the wiring ----------------------------------------------------------
+
+    @Test
+    fun `the receiver actually handles the action the alarm is sent with`() {
+        // The single point where the whole fix would die silently: the alarm
+        // fires, the receiver drops it on the action guard, nothing redraws.
+        assertThat(WidgetUpdateReceiver.HANDLED_ACTIONS)
+            .contains(WidgetUpdateReceiver.ACTION_ROLLOVER)
+    }
+}