fix(app): a sheet shows that more of it lies below the visible part

Found on the laptop: the Pomodoro profile's last switches (Do Not
Disturb among them) lay below the visible part of the sheet, and
nothing said so: the body scrolled right up to the pinned buttons.
Every sheet (SheetScaffold) now has, as Material 3 has scrolling
dialogs, a line above the footer while some of the body lies below and
one under the title while some lies above, and a scrollbar beside the
body, always shown on the desktop in place of the one Material shows
only while scrolling.

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

halkyon_learn_flutter/lib/src/core/widget/adaptive_sheet.dart

@@ -1,5 +1,7 @@
 import 'dart:math' as math;
 
+import 'package:flutter/foundation.dart';
+
 import 'package:halkyon_learn_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_learn_flutter/src/core/layout/window_size.dart';
 import 'package:lucide_icons_flutter/lucide_icons.dart';
@@ -126,6 +128,12 @@ class const SheetScaffold({
   /// The tallest a sheet grows as a dialog.
   static const dialogMaxHeight = 640.0;
 
+  /// The line under the title while some of the body lies above.
+  static const moreAboveKey = ValueKey('sheet-more-above');
+
+  /// The line above the footer while some of the body lies below.
+  static const moreBelowKey = ValueKey('sheet-more-below');
+
   @override
   Widget build(BuildContext context) {
     final media = MediaQuery.of(context);
@@ -142,9 +150,13 @@ class const SheetScaffold({
         // Loose, so a short body keeps the sheet short; tight at a fixed
         // height, so the footer sits at the bottom.
         if (!isDialog && heightFactor != null)
-          Expanded(child: SingleChildScrollView(child: body))
+          Expanded(
+            child: _ScrollingBody(body: body, fill: true, footer: footer),
+          )
         else
-          Flexible(child: SingleChildScrollView(child: body)),
+          Flexible(
+            child: _ScrollingBody(body: body, footer: footer),
+          ),
         if (footer case final footer?) ...[const SizedBox(height: 12), footer],
       ],
     );
@@ -238,3 +250,100 @@ class const _Header({
     ],
   );
 }
+
+/// The sheet's body, scrolling, with a scrollbar beside it (always shown on
+/// the desktop, where a wheel or touchpad scrolls without showing one) and,
+/// as Material 3 has scrolling dialogs, a line under the title while some of
+/// it lies above, and one above the [footer] while some of it lies below
+/// (decided 2026-10-07: a form's last switches went unnoticed below the
+/// buttons). [fill] takes the whole height given, rather than hugging the
+/// body.
+class const _ScrollingBody({
+  required final Widget body,
+  required final Widget? footer,
+  final bool fill = false,
+}) extends StatefulWidget {
+  @override
+  State<_ScrollingBody> createState() => _ScrollingBodyState();
+}
+
+const _bodyKey = ValueKey('sheet-body');
+
+class _ScrollingBodyState() extends State<_ScrollingBody> {
+  final _scroll = ScrollController();
+  var _moreAbove = false;
+  var _moreBelow = false;
+
+  void _update(ScrollMetrics metrics) {
+    final moreAbove = metrics.pixels > metrics.minScrollExtent;
+    final moreBelow = metrics.pixels < metrics.maxScrollExtent;
+    if (moreAbove != _moreAbove || moreBelow != _moreBelow) {
+      setState(() {
+        _moreAbove = moreAbove;
+        _moreBelow = moreBelow;
+      });
+    }
+  }
+
+  @override
+  void dispose() {
+    _scroll.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final desktop = switch (defaultTargetPlatform) {
+      TargetPlatform.linux ||
+      TargetPlatform.windows ||
+      TargetPlatform.macOS => true,
+      _ => false,
+    };
+    // The lines keep their room while hidden, so the body doesn't jump.
+    Widget line({required bool shown, required Key key}) =>
+        shown ? Divider(key: key, height: 1) : const SizedBox(height: 1);
+    final scrolling = NotificationListener<ScrollMetricsNotification>(
+      // Also when the body's size changes, as a section opens.
+      onNotification: (notification) {
+        _update(notification.metrics);
+        return false;
+      },
+      child: NotificationListener<ScrollUpdateNotification>(
+        onNotification: (notification) {
+          _update(notification.metrics);
+          return false;
+        },
+        child: Scrollbar(
+          controller: _scroll,
+          thumbVisibility: desktop,
+          // In place of the one Material's scroll behaviour adds on the
+          // desktop, shown only while scrolling.
+          child: ScrollConfiguration(
+            behavior: ScrollConfiguration.of(
+              context,
+            ).copyWith(scrollbars: false),
+            child: SingleChildScrollView(
+              controller: _scroll,
+              child: widget.body,
+            ),
+          ),
+        ),
+      ),
+    );
+    return Column(
+      mainAxisSize: MainAxisSize.min,
+      crossAxisAlignment: CrossAxisAlignment.stretch,
+      children: [
+        line(shown: _moreAbove, key: SheetScaffold.moreAboveKey),
+        // Keyed, so that a line coming or going beside it keeps the scroll
+        // view and its position.
+        if (widget.fill)
+          Expanded(key: _bodyKey, child: scrolling)
+        else
+          Flexible(key: _bodyKey, child: scrolling),
+        if (widget.footer != null)
+          line(shown: _moreBelow, key: SheetScaffold.moreBelowKey),
+      ],
+    );
+  }
+}

halkyon_learn_flutter/test/core/widget/adaptive_sheet_test.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:halkyon_learn_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_learn_flutter/src/core/widget/adaptive_sheet.dart';
@@ -103,4 +104,67 @@ void main() {
 
     expect(find.text('New studyset'), findsNothing);
   });
+
+  group('a body longer than the sheet', () {
+    final tall = Column(
+      children: [for (var i = 0; i < 40; i++) const Text('Line')],
+    );
+    Finder below() => find.byKey(SheetScaffold.moreBelowKey);
+    Finder above() => find.byKey(SheetScaffold.moreAboveKey);
+
+    testWidgets('shows a line above the buttons while more lies below, and '
+        'one under the title while more lies above', (tester) async {
+      // As a dialog: a modal bottom sheet takes a test's drag itself.
+      await open(tester, width: 1000, body: tall);
+
+      expect(below(), findsOneWidget);
+      expect(above(), findsNothing);
+
+      await tester.drag(
+        find.byType(SingleChildScrollView),
+        const Offset(0, -300),
+      );
+      await tester.pumpAndSettle();
+      expect(below(), findsOneWidget);
+      expect(above(), findsOneWidget);
+      expect(
+        tester
+            .state<ScrollableState>(
+              find.descendant(
+                of: find.byType(SingleChildScrollView),
+                matching: find.byType(Scrollable),
+              ),
+            )
+            .position
+            .pixels,
+        greaterThan(0),
+        reason: 'the line coming keeps where the body was scrolled to',
+      );
+
+      await tester.drag(
+        find.byType(SingleChildScrollView),
+        const Offset(0, -5000),
+      );
+      await tester.pumpAndSettle();
+      expect(below(), findsNothing);
+      expect(above(), findsOneWidget);
+    });
+
+    testWidgets('has a scrollbar, always shown on the desktop', (tester) async {
+      debugDefaultTargetPlatformOverride = TargetPlatform.linux;
+      addTearDown(() => debugDefaultTargetPlatformOverride = null);
+      await open(tester, width: 1000, body: tall);
+
+      final scrollbar = tester.widget<Scrollbar>(find.byType(Scrollbar));
+      expect(scrollbar.thumbVisibility, isTrue);
+      debugDefaultTargetPlatformOverride = null;
+    });
+  });
+
+  testWidgets('a body that fits shows no lines', (tester) async {
+    await open(tester, width: 400);
+
+    expect(find.byKey(SheetScaffold.moreBelowKey), findsNothing);
+    expect(find.byKey(SheetScaffold.moreAboveKey), findsNothing);
+  });
 }