fix(ui): on a phone, a snack bar stands above the screen's floating button

Below medium windows the frame's scaffold showed the screens' snack bars,
under the floating button of the screen's own scaffold, so a tap on a
snack bar's Undo hit the button instead (found 9 Nov 2026, Todo's Undo
after deleting the completed tasks). The screens get a messenger of their
own there, and their scaffolds lift the snack bars above their buttons, as
Material places them. From medium windows on the frame's scaffold still
shows them, once: a messenger shows a snack bar on each of its scaffolds
not inside another, as two panes side by side are. As the window grows
past a phone's, the phone's snack bars go, since a scaffold whose
messenger changes keeps the one it showed for good.

Whats-New: On a phone, a message's Undo is no longer hidden under the + button.
Whats-New-De: Auf dem Handy liegt das Rückgängig einer Meldung nicht mehr unter der +-Schaltfläche.
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/learn/halkyon_learn_flutter/test/core/router/app_shell_test.dart

@@ -147,8 +147,9 @@ void main() {
     expect(fadesInto(), scheme.surfaceContainerLow);
   });
 
-  testWidgets('a snack bar floats 8 above the navigation bar and its fade, '
+  testWidgets("a snack bar floats above the screen's floating button, "
-      '16 in from the sides; from medium windows on 560 wide, centred', (
+      'clear of the navigation bar and its fade, 16 in from the sides; from '
+      'medium windows on 560 wide, centred', (
     tester,
   ) async {
     tester.view.padding = const FakeViewPadding(bottom: 24);
@@ -171,12 +172,15 @@ void main() {
 
     await showApp(tester, width: 400);
     final narrow = await snackBar();
+    final button = tester.getRect(find.byType(FloatingActionButton));
     expect(narrow.left, 16);
     expect(narrow.right, 400 - 16);
-    // The fade's top, less 8.
+    // Above the Library's floating button, which would otherwise cover its
+    // action (found 9 Nov 2026), and so above the fade's top, less 8.
+    expect(narrow.bottom, lessThanOrEqualTo(button.top));
     expect(
       narrow.bottom,
-      900 - 24 - 64 - FlushNavigationBar.fadeHeight - 8,
+      lessThan(900 - 24 - 64 - FlushNavigationBar.fadeHeight - 8),
     );
 
     await showApp(tester);

packages/halkyon_ui/lib/src/widget/navigation_frame.dart

@@ -85,6 +85,15 @@ class const NavigationFrame({
 class _NavigationFrameState() extends State<NavigationFrame> {
   final _tint = ScreenTint();
 
+  /// The screens' own messenger below medium windows: their snack bars
+  /// show on the screens' own scaffolds, which lift them above their
+  /// floating buttons, as Material places them; on the frame's they lay
+  /// under the button, whose taps took the snack bar's Undo (found 9 Nov
+  /// 2026). From medium windows on the frame's scaffold shows them, once:
+  /// a messenger shows its snack bars on each of its scaffolds not inside
+  /// another, as two panes side by side are.
+  final _phoneMessenger = GlobalKey<ScaffoldMessengerState>();
+
   /// The latest wave sent by a tap; none before one.
   NavigationWave? _wave;
 
@@ -109,6 +118,13 @@ class _NavigationFrameState() extends State<NavigationFrame> {
   Widget build(BuildContext context) {
     final destinations = widget.destinations;
     if (context.windowSize.isAtLeastMedium) {
+      // The phone's messenger goes as the window grows, and a scaffold
+      // whose messenger changes would keep the snack bar it showed for
+      // good: they go first. Its scaffolds are below this frame, so they
+      // may be marked to build again while it builds.
+      _phoneMessenger.currentState
+        ?..clearSnackBars()
+        ..removeCurrentSnackBar();
       return ScreenTintScope(
         notifier: _tint,
         child: TintFrame(
@@ -218,7 +234,7 @@ class _NavigationFrameState() extends State<NavigationFrame> {
       // it keeps its size and lifts the field instead (Halkyon Learn's ADR
       // 0021, D69).
       resizeToAvoidBottomInset: false,
-      body: widget.child,
+      body: ScaffoldMessenger(key: _phoneMessenger, child: widget.child),
       // Without the bar, a strip as high as the system's own bar, in the
       // screen's background, keeps every screen inside clear of it.
       bottomNavigationBar: widget.hidesNavigationBar

packages/halkyon_ui/test/widget/navigation_frame_test.dart

@@ -250,6 +250,115 @@ void main() {
     );
   });
 
+  testWidgets("a screen's snack bar stands above its floating button, so "
+      'the button never takes its Undo (found 9 Nov 2026)', (tester) async {
+    tester.view.physicalSize = const Size(400, 800);
+    tester.view.devicePixelRatio = 1;
+    addTearDown(tester.view.reset);
+    var undone = 0;
+    var added = 0;
+    await tester.pumpWidget(
+      testApp(
+        NavigationFrame(
+          appName: 'Halkyon Test',
+          icon: icon,
+          accent: const Color(0xFFEC3E7C),
+          selectedIndex: 0,
+          onSelected: (_) {},
+          destinations: const [
+            (label: 'Home', icon: Icon(Icons.home), selectedIcon: null),
+            (label: 'Maps', icon: Icon(Icons.map), selectedIcon: null),
+          ],
+          child: Builder(
+            builder: (context) => Scaffold(
+              body: Center(
+                child: TextButton(
+                  onPressed: () => showUndoSnackBar(
+                    context,
+                    message: 'Task completed',
+                    onUndo: () => undone++,
+                  ),
+                  child: const Text('Complete'),
+                ),
+              ),
+              floatingActionButton: Padding(
+                padding: EdgeInsets.only(
+                  bottom: FlushNavigationBar.buttonLiftOf(context),
+                ),
+                child: FloatingActionButton(
+                  onPressed: () => added++,
+                  child: const Icon(Icons.add),
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.text('Complete'));
+    await tester.pumpAndSettle();
+
+    expect(
+      tester.getRect(find.text('Undo')).bottom,
+      lessThanOrEqualTo(
+        tester.getRect(find.byType(FloatingActionButton)).top,
+      ),
+    );
+    await tester.tap(find.text('Undo'));
+    expect((undone, added), (1, 0));
+  });
+
+  testWidgets("a screen's snack bar shown on a phone doesn't stay once the "
+      'window grows past it', (tester) async {
+    final screens = GlobalKey();
+    tester.view.physicalSize = const Size(400, 800);
+    tester.view.devicePixelRatio = 1;
+    addTearDown(tester.view.reset);
+    await tester.pumpWidget(
+      testApp(
+        NavigationFrame(
+          appName: 'Halkyon Test',
+          icon: icon,
+          accent: const Color(0xFFEC3E7C),
+          selectedIndex: 0,
+          onSelected: (_) {},
+          destinations: const [
+            (label: 'Home', icon: Icon(Icons.home), selectedIcon: null),
+            (label: 'Maps', icon: Icon(Icons.map), selectedIcon: null),
+          ],
+          // The screens keep their state as the frame's layout changes, as an
+          // app's router keeps its navigator by its key.
+          child: KeyedSubtree(
+            key: screens,
+            child: Builder(
+              builder: (context) => Scaffold(
+                body: Center(
+                  child: TextButton(
+                    onPressed: () => ScaffoldMessenger.of(context).showSnackBar(
+                      const SnackBar(content: Text('Archived')),
+                    ),
+                    child: const Text('Archive'),
+                  ),
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.tap(find.text('Archive'));
+    await tester.pumpAndSettle();
+    expect(find.text('Archived'), findsOneWidget);
+
+    tester.view.physicalSize = const Size(1200, 800);
+    await tester.pumpAndSettle();
+    await tester.pump(const Duration(seconds: 5));
+    await tester.pumpAndSettle();
+    expect(find.text('Archived'), findsNothing);
+  });
+
   testWidgets("outside the frame, content leaves room for the system's bar "
       'alone', (tester) async {
     tester.view.padding = const FakeViewPadding(bottom: 24);