[Markel15]

Symptom

Opening the Habits tab before midnight (e.g. at 23:30), leaving the app in the background without killing the process, and returning after midnight (e.g. at 00:20) left the screen in a stale / hybrid state:

• Header progress (completedToday / totalHabits) and card completion states still reflected the previous day.
• In NumericHabitCard, the selection dot and value counter stayed anchored on yesterday even if the card recomposed, and tapping +/- could apply yesterday's base todayValue to the new date.
• In HabitCard (boolean), the new day could remain treated as a future (disabled) cell in the weekly calendar or fail to scroll across a Sunday -> Monday rollover.

Root cause

There was no reactive source of truth for "today" across the Habits feature; LocalDate.now() was called independently in multiple places:

1. HabitViewModel.init called getHabitsWithStatus() only once. Its default parameter (date: LocalDate = LocalDate.now()) was evaluated at ViewModel creation time and stayed captured inside the UseCase flow for the entire lifetime of the ViewModel. Returning from the background without a DB emission never re-ran the combine block, and even on a DB emission the UseCase still computed isCompletedToday, todayValue and streaks against the captured date from the previous day.
2. NumericHabitCard keyed weekStart and selectedDate only on remember(weekOffset). Because weekOffset stayed 0 across midnight, selectedDate never reset to the new day.
3. HabitScreen passed LocalDate.now() directly to incrementNumericHabit / decrementNumericHabit while reading currentValue = habitWithStatus.todayValue (computed for the stale date).
4. HabitCard read LocalDate.now() directly during composition and only seeded rememberWeekCalendarState's firstVisibleWeekDate on initial composition.

Fix

Make today: LocalDate part of HabitUiState.Success and drive it from a lifecycle-aware StateFlow in HabitViewModel:

• HabitUiState.kt: add today: LocalDate to HabitUiState.Success.
• HabitViewModel.kt: implement DefaultLifecycleObserver and hold a _today = MutableStateFlow(LocalDate.now()) updated in onResume via refreshTodayIfNeeded(). Switch the UseCase subscription with _today.flatMapLatest { today -> getHabitsWithStatus(today).map { today to it } } so today and habits always emit together without an intermediate frame where the date has advanced but the statuses have not.
• HabitScreen.kt: register the ViewModel as a lifecycle observer via DisposableEffect and pass state.today down to HabitCard, NumericHabitCard and the today increment/decrement callbacks.
• NumericHabitCard.kt: accept today as a parameter and include it as a key in remember(today) / remember(today, weekOffset) for weekOffset, weekStart and selectedDate.
• HabitCard.kt: accept today as a parameter and scroll the WeekCalendar to today inside a LaunchedEffect(today) when the date advances.
• HabitViewModelTest.kt: add unit test covering the midnight rollover via refreshTodayIfNeeded().

Behavior note

Because _today is a MutableStateFlow, resuming the app on the same calendar day is a no-op (structural equality check prevents any emission or DB re-computation). Re-computation only happens when the date has actually changed while the app was in the background.