feat(todo): a sub-task opens in its task's sheet, back returning to it

A sub-task tapped in a task's sheet opened a sheet of its own over it,
so Grocery List > Dairy > Milk stacked three. It now takes the sheet's
place, as the steps of the sheet's path already did (TaskTile's onOpen),
and the sheet keeps the tasks it showed: the system's back returns to
the last. Back is caught with a BackButtonListener, not a PopScope,
which would also hold back a tap beside the sheet, a swipe and Escape:
those close the sheet whole. A picker open over the sheet is closed by
back first.

Whats-New: A sub-task opens in its task's sheet instead of over it; back returns to the task.
Whats-New-De: Eine Unteraufgabe öffnet sich im Blatt ihrer Aufgabe statt darüber; Zurück führt zur Aufgabe.

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

@@ -145,7 +145,11 @@ class const _SheetScroll({
       controller != oldWidget.controller;
 }
 
-/// A sheet's task, which a step of its path replaces in place.
+/// A sheet's task, which a step of its path or a sub-task replaces in place
+/// (the developer's review, 4 Nov 2026: a sub-task had opened a sheet over
+/// the sheet). The tasks it showed before are kept, so the system's back
+/// returns to the last; every other way of closing it (a tap beside it, a
+/// swipe down, Escape) closes it whole.
 class const _TaskSheetHost({required final String id}) extends StatefulWidget {
   @override
   State<_TaskSheetHost> createState() => _TaskSheetHostState();
@@ -154,12 +158,45 @@ class const _TaskSheetHost({required final String id}) extends StatefulWidget {
 class _TaskSheetHostState() extends State<_TaskSheetHost> {
   late var _id = widget.id;
 
+  /// The tasks shown before [_id], the last most recently.
+  final _history = <String>[];
+
+  /// Shows [id]; one shown before is gone back to, the steps after it
+  /// forgotten.
+  void _show(String id) {
+    if (id == _id) {
+      return;
+    }
+    setState(() {
+      final before = _history.indexOf(id);
+      if (before >= 0) {
+        _history.removeRange(before, _history.length);
+      } else {
+        _history.add(_id);
+      }
+      _id = id;
+    });
+  }
+
+  /// Back from the system: the task shown before, while there is one and
+  /// nothing is open over the sheet (a picker, which back closes first).
+  Future<bool> _back() async {
+    if (_history.isEmpty || !(ModalRoute.of(context)?.isCurrent ?? false)) {
+      return false;
+    }
+    setState(() => _id = _history.removeLast());
+    return true;
+  }
+
   @override
-  Widget build(BuildContext context) => _TaskSheet(
+  Widget build(BuildContext context) {
-    key: ValueKey(_id),
+    final sheet = _TaskSheet(key: ValueKey(_id), id: _id, onOpen: _show);
-    id: _id,
+    // Not a PopScope: that would hold back a tap beside the sheet and
-    onOpen: (id) => setState(() => _id = id),
+    // Escape too, which close it whole. Back alone goes through the router.
-  );
+    return Router.maybeOf(context) == null
+        ? sheet
+        : BackButtonListener(onBackButtonPressed: _back, child: sheet);
+  }
 }
 
 /// The task open beside the list from expanded windows on (ADR 0033, D2,
@@ -759,7 +796,11 @@ class _TaskSheetState() extends State<_TaskSheet> {
                   ),
                   for (final sub in subTasks)
                     if (!(content.hideDoneSubTasks && sub.content.completed))
-                      TaskTile(task: sub, selectable: false),
+                      TaskTile(
+                        task: sub,
+                        selectable: false,
+                        onOpen: (id) => _open(context, id),
+                      ),
                 ],
                 TextButton.icon(
                   icon: const Icon(LucideIcons.plus),

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

@@ -46,6 +46,10 @@ class const TaskTile({
   final bool selectable = true,
   final bool later = false,
   final TaskRowPosition position = TaskRowPosition.alone,
+
+  /// Opens the task in place of the one showing, as a sheet's sub-tasks
+  /// do; by default a tap opens the task's sheet.
+  final ValueChanged<String>? onOpen,
   super.key,
 }) extends StatelessWidget {
   @override
@@ -252,7 +256,10 @@ class const TaskTile({
       ),
       onTap: selection != null && selection.active
           ? () => selection.toggle(task.id)
-          : () => openTaskSheet(context, task.id),
+          : switch (onOpen) {
+              final open? => () => open(task.id),
+              null => () => openTaskSheet(context, task.id),
+            },
     );
     if (selection == null) {
       return _boxed(context, tile);

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

@@ -2184,6 +2184,44 @@ void main() {
       );
     });
 
+    testWidgets("a sub-task opens in its task's sheet, not in another over "
+        'it; back returns to the task, a tap beside closes it (the '
+        "developer's review, 4 Nov 2026)", (tester) async {
+      await start(tester, usedBefore: true);
+      final list = await tester.runAsync(
+        () =>
+            tasks().add(TaskContent(title: 'Grocery List', projectId: inbox())),
+      );
+      final dairy = await tester.runAsync(
+        () => tasks().add(
+          TaskContent(title: 'Dairy', projectId: inbox(), parentId: list),
+        ),
+      );
+      await seed(
+        tester,
+        TaskContent(title: 'Milk', projectId: inbox(), parentId: dairy),
+      );
+      await goTab(tester, 'Inbox');
+
+      await tester.tap(find.text('Grocery List').first);
+      await tester.pumpAndSettle();
+      await tester.tap(find.text('Dairy').last);
+      await tester.pumpAndSettle();
+      await tester.tap(find.text('Milk').last);
+      await tester.pumpAndSettle();
+      expect(find.byType(BottomSheet), findsOneWidget);
+      expect(find.widgetWithText(TextField, 'Milk'), findsOneWidget);
+
+      await tester.binding.handlePopRoute();
+      await tester.pumpAndSettle();
+      expect(find.byType(BottomSheet), findsOneWidget);
+      expect(find.widgetWithText(TextField, 'Dairy'), findsOneWidget);
+
+      await tester.tapAt(const Offset(20, 20));
+      await tester.pumpAndSettle();
+      expect(find.byType(BottomSheet), findsNothing);
+    });
+
     testWidgets("a task's menu says when it was added, duplicates it with its "
         'sub-tasks and hides its done ones, on every device', (tester) async {
       await start(tester, usedBefore: true);