feat(todo): Upcoming's tasks dragged onto another day (ADR 0033, D16)

A row of Upcoming, overdue ones too, held and dropped on another day
moves there, its time, duration and recurrence kept; Moved to … offers
Undo. The tint over a day a drag hovers is laid beside the rows, not
around them, which paint on the Material beneath.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/lib/l10n/app_de.arb

@@ -167,5 +167,6 @@
   "quickAddTokenHint": "Tippe ein hervorgehobenes Wort an, um es als Text zu behalten.",
   "deadlineTitle": "Frist",
   "orderChanged": "Reihenfolge geändert",
-  "addTaskCancel": "Hier ablegen zum Abbrechen"
+  "addTaskCancel": "Hier ablegen zum Abbrechen",
+  "taskMovedTo": "Verschoben auf {day}"
 }

apps/todo/halkyon_todo_flutter/lib/l10n/app_en.arb

@@ -457,5 +457,14 @@
   "addTaskCancel": "Drop here to cancel",
   "@addTaskCancel": {
     "description": "The × left in the add button's place while it is dragged (D16)."
+  },
+  "taskMovedTo": "Moved to {day}",
+  "@taskMovedTo": {
+    "description": "Snack bar after a task was dragged onto another day in Upcoming (ADR 0033, D16).",
+    "placeholders": {
+      "day": {
+        "type": "String"
+      }
+    }
   }
 }

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations.dart

@@ -1107,6 +1107,12 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Drop here to cancel'**
   String get addTaskCancel;
+
+  /// Snack bar after a task was dragged onto another day in Upcoming (ADR 0033, D16).
+  ///
+  /// In en, this message translates to:
+  /// **'Moved to {day}'**
+  String taskMovedTo(String day);
 }
 
 class _AppLocalizationsDelegate

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_de.dart

@@ -713,4 +713,9 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get addTaskCancel => 'Hier ablegen zum Abbrechen';
+
+  @override
+  String taskMovedTo(String day) {
+    return 'Verschoben auf $day';
+  }
 }

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_en.dart

@@ -712,6 +712,11 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get addTaskCancel => 'Drop here to cancel';
+
+  @override
+  String taskMovedTo(String day) {
+    return 'Moved to $day';
+  }
 }
 
 /// The translations for English, as used in the United Kingdom (`en_GB`).

apps/todo/halkyon_todo_flutter/lib/src/feature/task/widget/task_drag.dart

@@ -1,12 +1,18 @@
 import 'dart:async';
 
+import 'package:clock/clock.dart';
+import 'package:flutter_bloc/flutter_bloc.dart';
+
 import 'package:halkyon_app/halkyon_app.dart';
 import 'package:halkyon_todo_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_todo_flutter/src/app/todo_dependencies.dart';
 import 'package:halkyon_todo_flutter/src/core/privacy/todo_keys.dart';
+import 'package:halkyon_todo_flutter/src/feature/task/bloc/todo_cubit.dart';
+import 'package:halkyon_todo_flutter/src/feature/task/model/task.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/model/task_views.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/quick_add_sheet.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/task_selection.dart';
+import 'package:halkyon_todo_flutter/src/feature/task/widget/task_texts.dart';
 import 'package:halkyon_ui/halkyon_ui.dart';
 import 'package:lucide_icons_flutter/lucide_icons.dart';
 import 'package:material_ui/material_ui.dart';
@@ -264,6 +270,76 @@ class _DraggableTaskState() extends State<DraggableTask> {
   }
 }
 
+/// A day of Upcoming (D16): the add button dropped on it adds a task for
+/// [day]; a task's row moves there, its time, duration and recurrence
+/// kept, which can be undone for a moment. Tinted while either is over it.
+class const DayDropArea({
+  required final Day day,
+  required final Widget child,
+  super.key,
+}) extends StatelessWidget {
+  Future<void> _move(BuildContext context, String id) async {
+    final l10n = AppLocalizations.of(context);
+    final messenger = ScaffoldMessenger.of(context);
+    final tasks = PersonScope.of<TodoPerson>(context).tasks;
+    final task = context
+        .read<TodoCubit>()
+        .state
+        .tasks
+        .where((task) => task.id == id)
+        .firstOrNull;
+    if (task == null || task.content.date?.day == day) {
+      return;
+    }
+    final content = task.content;
+    final moved = content.copyWith(
+      date: () => content.date?.on(day) ?? TaskDate(day),
+    );
+    try {
+      await tasks.update(id, moved);
+    } on KeysMissing {
+      messenger.showSnackBar(SnackBar(content: Text(l10n.keysMissing)));
+      return;
+    }
+    if (context.mounted) {
+      showUndoSnackBar(
+        context,
+        message: l10n.taskMovedTo(
+          dayText(context, day, Day.of(clock.now())),
+        ),
+        onUndo: () => unawaited(tasks.update(id, content)),
+      );
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) => DragTarget<Object>(
+    onWillAcceptWithDetails: (details) =>
+        details.data is NewTaskDrag || details.data is TaskDrag,
+    onAcceptWithDetails: (details) => switch (details.data) {
+      TaskDrag(:final id) => unawaited(_move(context, id)),
+      _ => unawaited(showQuickAdd(context, day: day)),
+    },
+    // Laid over the rows beside them, not around them: the rows paint on
+    // the Material beneath.
+    builder: (context, candidates, _) => Stack(
+      children: [
+        child,
+        if (candidates.isNotEmpty)
+          Positioned.fill(
+            child: IgnorePointer(
+              child: ColoredBox(
+                color: Theme.of(
+                  context,
+                ).colorScheme.primary.withValues(alpha: 0.08),
+              ),
+            ),
+          ),
+      ],
+    ),
+  );
+}
+
 /// A list's area the add button can be dropped on (D16), [onDrop] opening
 /// quick add for it; tinted while the button is over it.
 class const NewTaskDropArea({
@@ -274,11 +350,22 @@ class const NewTaskDropArea({
   @override
   Widget build(BuildContext context) => DragTarget<NewTaskDrag>(
     onAcceptWithDetails: (_) => onDrop(),
-    builder: (context, candidates, _) => ColoredBox(
-      color: candidates.isEmpty
-          ? Colors.transparent
-          : Theme.of(context).colorScheme.primary.withValues(alpha: 0.08),
-      child: child,
+    // Laid over the rows beside them, not around them: the rows paint on
+    // the Material beneath.
+    builder: (context, candidates, _) => Stack(
+      children: [
+        child,
+        if (candidates.isNotEmpty)
+          Positioned.fill(
+            child: IgnorePointer(
+              child: ColoredBox(
+                color: Theme.of(
+                  context,
+                ).colorScheme.primary.withValues(alpha: 0.08),
+              ),
+            ),
+          ),
+      ],
     ),
   );
 }

apps/todo/halkyon_todo_flutter/lib/src/tabs/upcoming_tab.dart

@@ -158,14 +158,18 @@ class _UpcomingTabState() extends State<UpcomingTab> {
                               ),
                             ),
                             for (final task in overdue)
-                              TaskTile(task: task, showProject: true),
+                              DraggableTask(
+                                task: task,
+                                child: TaskTile(task: task, showProject: true),
+                              ),
                           ],
                           // The add button dropped on a day adds a task
-                          // for it (D16).
+                          // for it, a row held and dropped there moves to
+                          // it (D16).
                           for (final (day, tasks) in days)
-                            NewTaskDropArea(
+                            DayDropArea(
                               key: _keyOf(day),
-                              onDrop: () => showQuickAdd(context, day: day),
+                              day: day,
                               child: Column(
                                 crossAxisAlignment: CrossAxisAlignment.stretch,
                                 children: [
@@ -182,7 +186,13 @@ class _UpcomingTabState() extends State<UpcomingTab> {
                                     ),
                                   ),
                                   for (final task in tasks)
-                                    TaskTile(task: task, showProject: true),
+                                    DraggableTask(
+                                      task: task,
+                                      child: TaskTile(
+                                        task: task,
+                                        showProject: true,
+                                      ),
+                                    ),
                                 ],
                               ),
                             ),

apps/todo/halkyon_todo_flutter/test/app/todo_app_test.dart

@@ -1374,6 +1374,40 @@ void main() {
         expect(find.byKey(quickAddTitleKey), findsNothing);
       });
 
+      testWidgets('a row of Upcoming held and dropped on another day moves '
+          'there, its time kept, which can be undone', (tester) async {
+        final today = Day.of(DateTime.now());
+        await start(tester, usedBefore: true);
+        await seed(
+          tester,
+          TaskContent(
+            title: 'Water plants',
+            projectId: inbox(),
+            date: TaskDate(today, time: 23 * 60 + 30),
+          ),
+        );
+        await goTab(tester, 'Upcoming');
+        final day = find.byTooltip(RegExp('^Add a task on .*Tomorrow'));
+
+        await dragFrom(
+          tester,
+          tester.getCenter(row('Water plants')),
+          tester.getCenter(day) - const Offset(200, 0),
+        );
+
+        expect(
+          (await stored(tester))['Water plants']!.date,
+          TaskDate(today.plus(1), time: 23 * 60 + 30),
+        );
+        expect(find.textContaining('Moved to'), findsOneWidget);
+        await tester.tap(find.text('Undo'));
+        await tester.pumpAndSettle();
+        expect(
+          (await stored(tester))['Water plants']!.date,
+          TaskDate(today, time: 23 * 60 + 30),
+        );
+      });
+
       testWidgets('the add button dropped on a day in Upcoming adds a task '
           'for that day', (tester) async {
         final tomorrow = Day.of(DateTime.now()).plus(1);