feat(todo): a task's sub-tasks held and dragged into another order in its sheet

As a project's rows are (D16): a long press on a sub-task in the task's
sheet or pane lifts it, and dropped on the upper or lower half of
another it goes there, still the task's sub-task (the developer's
review, 10 Nov 2026).

Whats-New: In a task's details, hold a sub-task to drag it into another order.
Whats-New-De: In den Details einer Aufgabe hältst du eine Unteraufgabe gedrückt, um sie in eine andere Reihenfolge zu ziehen.

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/src/feature/task/widget/task_sheet.dart

@@ -27,6 +27,7 @@ import 'package:halkyon_todo_flutter/src/feature/task/model/task_views.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/date_picker.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/quick_add_sheet.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/repeat_picker.dart';
+import 'package:halkyon_todo_flutter/src/feature/task/widget/task_drag.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/task_pickers.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/task_texts.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/task_tile.dart';
@@ -782,6 +783,10 @@ class _TaskSheetState() extends State<_TaskSheet> {
     final today = Day.of(clock.now());
     final project = state.projectOf(content.projectId);
     final subTasks = subTasksOf(state.tasks, task.id);
+    final shownSubTasks = [
+      for (final sub in subTasks)
+        if (!(content.hideDoneSubTasks && sub.content.completed)) sub,
+    ];
     final labels = {
       for (final label in state.labels) label.id: label.content!.name,
     };
@@ -1009,13 +1014,17 @@ class _TaskSheetState() extends State<_TaskSheet> {
                       ),
                     ),
                   ),
-                  for (final sub in subTasks)
+                  for (final (index, sub) in shownSubTasks.indexed)
-                    if (!(content.hideDoneSubTasks && sub.content.completed))
+                    _placedSubTask(
+                      task,
+                      shownSubTasks,
+                      index,
                       TaskTile(
                         task: sub,
                         selectable: false,
                         onOpen: (id) => _open(context, id),
                       ),
+                    ),
                 ],
                 TextButton.icon(
                   icon: const Icon(LucideIcons.plus),
@@ -1036,6 +1045,39 @@ class _TaskSheetState() extends State<_TaskSheet> {
     );
   }
 
+  /// The [index]th of [task]'s [subTasks] shown, as [child], held to be
+  /// dragged to another place among them and taking a drop above or below
+  /// it, staying [task]'s sub-task (the developer's review, 10 Nov 2026).
+  static Widget _placedSubTask(
+    OpenTask task,
+    List<OpenTask> subTasks,
+    int index,
+    Widget child,
+  ) {
+    final sub = subTasks[index];
+    final sectionId = sub.content.sectionId;
+    return TaskDropTarget(
+      key: ValueKey('drop-${sub.id}'),
+      projectId: sub.content.projectId,
+      excluded: sub.id,
+      above: DropSpot(
+        sectionId: sectionId,
+        parentId: task.id,
+        after: index > 0 ? subTasks[index - 1].content.position : null,
+        before: sub.content.position,
+      ),
+      below: DropSpot(
+        sectionId: sectionId,
+        parentId: task.id,
+        after: sub.content.position,
+        before: index + 1 < subTasks.length
+            ? subTasks[index + 1].content.position
+            : null,
+      ),
+      child: DraggableTask(task: sub, child: child),
+    );
+  }
+
   /// Asks for the task's reminders and writes them, asking first to
   /// remind where nothing was to be reminded of before.
   Future<void> _pickReminders(BuildContext context, TaskContent content) async {

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

@@ -5614,6 +5614,53 @@ void main() {
                 .compareTo(tester.getCenter(row(b)).dy),
           );
 
+      testWidgets("in a task's sheet, a sub-task held and dropped on the "
+          'upper half of another goes before it, still its sub-task (the '
+          "developer's review, 10 Nov 2026)", (tester) async {
+        await start(tester, usedBefore: true);
+        final pack = (await tester.runAsync(
+          () => tasks().add(TaskContent(title: 'Pack', projectId: inbox())),
+        ))!;
+        for (final (index, title) in ['Alpha', 'Bravo', 'Charlie'].indexed) {
+          await seed(
+            tester,
+            TaskContent(
+              title: title,
+              projectId: inbox(),
+              parentId: pack,
+              position: index.toDouble(),
+            ),
+          );
+        }
+        await goTab(tester, 'Inbox');
+        await tester.tap(find.text('Pack'));
+        await tester.pumpAndSettle();
+        await tester.drag(
+          find.byKey(taskSheetHandleKey),
+          const Offset(0, -700),
+        );
+        await tester.pumpAndSettle();
+        Finder inSheet(String title) => find.descendant(
+          of: find.byType(DraggableScrollableSheet),
+          matching: row(title),
+        );
+
+        await dragFrom(
+          tester,
+          tester.getCenter(inSheet('Charlie')),
+          tester.getTopLeft(inSheet('Alpha')) + const Offset(100, 6),
+        );
+
+        final after = await stored(tester);
+        expect(after['Charlie']!.position, lessThan(after['Alpha']!.position));
+        expect(after['Charlie']!.parentId, pack);
+        final ys = [
+          for (final title in ['Charlie', 'Alpha', 'Bravo'])
+            tester.getCenter(inSheet(title)).dy,
+        ];
+        expect(ys, orderedEquals([...ys]..sort()));
+      });
+
       testWidgets('a row held and dropped on the upper half of another goes '
           'before it, which can be undone', (tester) async {
         await inboxOf(tester, ['Alpha', 'Bravo', 'Charlie']);