fix(widget): refresh from setter, serialized with a mutex

The debounced observer was unreliable: drop(1) plus the ViewModel lifecycle
could swallow a change after re-entering the app, so a single range change
sometimes didn't refresh the widget. Refresh directly from the settings
setters instead (always fires on a real change), serialized through a mutex
so a rapid flip-and-flip-back can't run two updateAll calls at once and
strand the widget on the intermediate value.

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

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

@@ -19,25 +19,18 @@ import de.jeanlucmakiola.calendula.ui.agenda.AgendaRange
 import de.jeanlucmakiola.calendula.ui.common.CalendarView
 import de.jeanlucmakiola.calendula.widget.agenda.AgendaWidget
 import de.jeanlucmakiola.calendula.widget.month.MonthWidget
-import kotlinx.coroutines.FlowPreview
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.catch
 import kotlinx.coroutines.flow.combine
-import kotlinx.coroutines.flow.debounce
-import kotlinx.coroutines.flow.distinctUntilChanged
-import kotlinx.coroutines.flow.drop
-import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.map
-import kotlinx.coroutines.flow.onEach
 import kotlinx.coroutines.flow.stateIn
 import kotlinx.coroutines.launch
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
 import javax.inject.Inject
 
-private const val WIDGET_REFRESH_DEBOUNCE_MS = 350L
-
-@OptIn(FlowPreview::class)
 @HiltViewModel
 class SettingsViewModel @Inject constructor(
     private val prefs: SettingsPrefs,
@@ -123,30 +116,11 @@ class SettingsViewModel @Inject constructor(
             initialValue = SettingsUiState(dynamicColorAvailable = dynamicColorAvailable),
         )
 
-    init {
-        // Push widget redraws when their backing prefs change, debounced so a
-        // rapid flip-and-flip-back settles to one update of the final value
-        // (two racing updateAll calls can otherwise leave the widget stale).
-        // drop(1) skips the value present at startup — nothing to refresh yet.
-        prefs.agendaWidgetRange
-            .drop(1)
-            .debounce(WIDGET_REFRESH_DEBOUNCE_MS)
-            .distinctUntilChanged()
-            .onEach { AgendaWidget().updateAll(appContext) }
-            .launchIn(viewModelScope)
-
-        // Both widgets depend on the week start (month weekday header; the
-        // agenda widget's "this week" range).
-        prefs.weekStart
-            .drop(1)
-            .debounce(WIDGET_REFRESH_DEBOUNCE_MS)
-            .distinctUntilChanged()
-            .onEach {
-                AgendaWidget().updateAll(appContext)
-                MonthWidget().updateAll(appContext)
-            }
-            .launchIn(viewModelScope)
-    }
+    // Serialises widget redraws so a rapid flip-and-flip-back can't run two
+    // updateAll calls at once — concurrent calls coalesce around a stale read
+    // and can leave the widget on the intermediate value. Held sequentially,
+    // the last call reads the final committed pref and renders it.
+    private val widgetRefreshMutex = Mutex()
 
     private data class ReminderDefaults(
         val allowColor: Boolean,
@@ -179,7 +153,15 @@ class SettingsViewModel @Inject constructor(
     }
 
     fun setWeekStart(pref: WeekStartPref) {
-        viewModelScope.launch { prefs.setWeekStart(pref) }
+        viewModelScope.launch {
+            prefs.setWeekStart(pref)
+            // Both widgets depend on the week start (month weekday header; the
+            // agenda widget's "this week" range).
+            widgetRefreshMutex.withLock {
+                AgendaWidget().updateAll(appContext)
+                MonthWidget().updateAll(appContext)
+            }
+        }
     }
 
     fun setAgendaScreenRange(range: AgendaRange) {
@@ -187,7 +169,10 @@ class SettingsViewModel @Inject constructor(
     }
 
     fun setAgendaWidgetRange(range: AgendaRange) {
-        viewModelScope.launch { prefs.setAgendaWidgetRange(range) }
+        viewModelScope.launch {
+            prefs.setAgendaWidgetRange(range)
+            widgetRefreshMutex.withLock { AgendaWidget().updateAll(appContext) }
+        }
     }
 
     fun setAgendaShowRangeBar(enabled: Boolean) {