Fix reverse cases for App Bar scrolled under behavior (#101460)

packages/flutter/lib/src/material/app_bar.dart

@@ -737,24 +737,24 @@ class _AppBarState extends State<AppBar> {
   static const double _defaultElevation = 4.0;
   static const Color _defaultShadowColor = Color(0xFF000000);
 
-  ScrollNotificationObserverState? _scrollNotificationObserver;
+  ScrollMetricsNotificationObserverState? _scrollMetricsNotificationObserver;
   bool _scrolledUnder = false;
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    if (_scrollNotificationObserver != null)
-      _scrollNotificationObserver!.removeListener(_handleScrollNotification);
-    _scrollNotificationObserver = ScrollNotificationObserver.of(context);
-    if (_scrollNotificationObserver != null)
-      _scrollNotificationObserver!.addListener(_handleScrollNotification);
+    if (_scrollMetricsNotificationObserver != null)
+      _scrollMetricsNotificationObserver!.removeListener(_handleScrollMetricsNotification);
+    _scrollMetricsNotificationObserver = ScrollMetricsNotificationObserver.of(context);
+    if (_scrollMetricsNotificationObserver != null)
+      _scrollMetricsNotificationObserver!.addListener(_handleScrollMetricsNotification);
   }
 
   @override
   void dispose() {
-    if (_scrollNotificationObserver != null) {
-      _scrollNotificationObserver!.removeListener(_handleScrollNotification);
-      _scrollNotificationObserver = null;
+    if (_scrollMetricsNotificationObserver != null) {
+      _scrollMetricsNotificationObserver!.removeListener(_handleScrollMetricsNotification);
+      _scrollMetricsNotificationObserver = null;
     }
     super.dispose();
   }
@@ -767,18 +767,34 @@ class _AppBarState extends State<AppBar> {
     Scaffold.of(context).openEndDrawer();
   }
 
-  void _handleScrollNotification(ScrollNotification notification) {
-    if (notification is ScrollUpdateNotification) {
-      final bool oldScrolledUnder = _scrolledUnder;
-      _scrolledUnder = notification.depth == 0
-          && notification.metrics.extentBefore > 0
-          && notification.metrics.axis == Axis.vertical;
-      if (_scrolledUnder != oldScrolledUnder) {
-        setState(() {
-          // React to a change in MaterialState.scrolledUnder
-        });
+  void _handleScrollMetricsNotification(ScrollMetricsNotification notification) {
+    final bool oldScrolledUnder = _scrolledUnder;
+    final ScrollMetrics metrics = notification.metrics;
+
+    if (notification.depth != 0) {
+      _scrolledUnder = false;
+    } else {
+      switch (metrics.axisDirection) {
+        case AxisDirection.up:
+          // Scroll view is reversed
+          _scrolledUnder = metrics.extentAfter > 0;
+          break;
+        case AxisDirection.down:
+          _scrolledUnder = metrics.extentBefore > 0;
+          break;
+        case AxisDirection.right:
+        case AxisDirection.left:
+        // Scrolled under is only supported in the vertical axis.
+          _scrolledUnder = false;
+          break;
       }
     }
+
+    if (_scrolledUnder != oldScrolledUnder) {
+      setState(() {
+        // React to a change in MaterialState.scrolledUnder
+      });
+    }
   }
 
   Color _resolveColor(Set<MaterialState> states, Color? widgetColor, Color? themeColor, Color defaultColor) {

packages/flutter/lib/src/material/scaffold.dart

@@ -3096,7 +3096,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin, Resto
     return _ScaffoldScope(
       hasDrawer: hasDrawer,
       geometryNotifier: _geometryNotifier,
-      child: ScrollNotificationObserver(
+      child: ScrollMetricsNotificationObserver(
         child: Material(
           color: widget.backgroundColor ?? themeData.scaffoldBackgroundColor,
           child: AnimatedBuilder(animation: _floatingActionButtonMoveController, builder: (BuildContext context, Widget? child) {

packages/flutter/lib/src/widgets/scroll_notification_observer.dart

@@ -9,6 +9,7 @@ import 'package:flutter/foundation.dart';
 import 'framework.dart';
 import 'notification_listener.dart';
 import 'scroll_notification.dart';
+import 'scroll_position.dart';
 
 /// A [ScrollNotification] listener for [ScrollNotificationObserver].
 ///
@@ -172,3 +173,170 @@ class ScrollNotificationObserverState extends State<ScrollNotificationObserver>
     super.dispose();
   }
 }
+
+/// A [ScrollMetricsNotification] listener for [ScrollMetricsNotificationObserver].
+///
+/// [ScrollMetricsNotificationObserver] is similar to
+/// [NotificationListener]. It supports a listener list instead of
+/// just a single listener and its listeners run unconditionally, they
+/// do not require a gating boolean return value.
+typedef ScrollMetricsNotificationCallback = void Function(ScrollMetricsNotification notification);
+
+class _ScrollMetricsNotificationObserverScope extends InheritedWidget {
+  const _ScrollMetricsNotificationObserverScope({
+    Key? key,
+    required Widget child,
+    required ScrollMetricsNotificationObserverState scrollMetricsNotificationObserverState,
+  }) : _scrollMetricsNotificationObserverState = scrollMetricsNotificationObserverState,
+        super(key: key, child: child);
+
+  final ScrollMetricsNotificationObserverState  _scrollMetricsNotificationObserverState;
+
+  @override
+  bool updateShouldNotify(_ScrollMetricsNotificationObserverScope old) {
+    return _scrollMetricsNotificationObserverState != old._scrollMetricsNotificationObserverState;
+  }
+}
+
+class _MetricsListenerEntry extends LinkedListEntry<_MetricsListenerEntry> {
+  _MetricsListenerEntry(this.listener);
+  final ScrollMetricsNotificationCallback listener;
+}
+
+/// Notifies its listeners when a descendant ScrollMetrics are
+/// initialized or updated.
+///
+/// To add a listener to a [ScrollMetricsNotificationObserver] ancestor:
+/// ```dart
+/// void listener(ScrollMetricsNotification notification) {
+///   // Do something, maybe setState()
+/// }
+/// ScrollMetricsNotificationObserver.of(context).addListener(listener)
+/// ```
+///
+/// To remove the listener from a [ScrollMetricsNotificationObserver] ancestor:
+/// ```dart
+/// ScrollMetricsNotificationObserver.of(context).removeListener(listener);
+/// ```
+///
+/// Stateful widgets that share an ancestor [ScrollMetricsNotificationObserver]
+/// typically add a listener in [State.didChangeDependencies] (removing the old
+/// one if necessary) and remove the listener in their [State.dispose] method.
+///
+/// This widget is similar to [NotificationListener]. It supports
+/// a listener list instead of just a single listener and its listeners
+/// run unconditionally, they do not require a gating boolean return value.
+class ScrollMetricsNotificationObserver extends StatefulWidget {
+  /// Create a [ScrollMetricsNotificationObserver].
+  ///
+  /// The [child] parameter must not be null.
+  const ScrollMetricsNotificationObserver({
+    Key? key,
+    required this.child,
+  }) : assert(child != null), super(key: key);
+
+  /// The subtree below this widget.
+  final Widget child;
+
+  /// The closest instance of this class that encloses the given context.
+  ///
+  /// If there is no enclosing [ScrollMetricsNotificationObserver] widget, then
+  /// null is returned.
+  static ScrollMetricsNotificationObserverState? of(BuildContext context) {
+    return context.dependOnInheritedWidgetOfExactType<_ScrollMetricsNotificationObserverScope>()?._scrollMetricsNotificationObserverState;
+  }
+
+  @override
+  ScrollMetricsNotificationObserverState createState() => ScrollMetricsNotificationObserverState();
+}
+
+/// The listener list state for a [ScrollMetricsNotificationObserver] returned
+/// by [ScrollMetricsNotificationObserver.of].
+///
+/// [ScrollMetricsNotificationObserver] is similar to
+/// [NotificationListener]. It supports a listener list instead of
+/// just a single listener and its listeners run unconditionally, they
+/// do not require a gating boolean return value.
+class ScrollMetricsNotificationObserverState extends State<ScrollMetricsNotificationObserver> {
+  LinkedList<_MetricsListenerEntry>? _listeners = LinkedList<_MetricsListenerEntry>();
+
+  bool _debugAssertNotDisposed() {
+    assert(() {
+      if (_listeners == null) {
+        throw FlutterError(
+          'A $runtimeType was used after being disposed.\n'
+              'Once you have called dispose() on a $runtimeType, it can no longer be used.',
+        );
+      }
+      return true;
+    }());
+    return true;
+  }
+
+  /// Add a [ScrollMetricsNotificationCallback] that will be called each time
+  /// a descendant scrolls.
+  void addListener(ScrollMetricsNotificationCallback listener) {
+    assert(_debugAssertNotDisposed());
+    _listeners!.add(_MetricsListenerEntry(listener));
+  }
+
+  /// Remove the specified [ScrollMetricsNotificationCallback].
+  void removeListener(ScrollMetricsNotificationCallback listener) {
+    assert(_debugAssertNotDisposed());
+    for (final _MetricsListenerEntry entry in _listeners!) {
+      if (entry.listener == listener) {
+        entry.unlink();
+        return;
+      }
+    }
+  }
+
+  void _notifyListeners(ScrollMetricsNotification notification) {
+    assert(_debugAssertNotDisposed());
+    if (_listeners!.isEmpty)
+      return;
+
+    final List<_MetricsListenerEntry> localListeners = List<_MetricsListenerEntry>.of(_listeners!);
+    for (final _MetricsListenerEntry entry in localListeners) {
+      try {
+        if (entry.list != null)
+          entry.listener(notification);
+      } catch (exception, stack) {
+        FlutterError.reportError(FlutterErrorDetails(
+          exception: exception,
+          stack: stack,
+          library: 'widget library',
+          context: ErrorDescription('while dispatching notifications for $runtimeType'),
+          informationCollector: () => <DiagnosticsNode>[
+            DiagnosticsProperty<ScrollMetricsNotificationObserverState>(
+              'The $runtimeType sending notification was',
+              this,
+              style: DiagnosticsTreeStyle.errorProperty,
+            ),
+          ],
+        ));
+      }
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return NotificationListener<ScrollMetricsNotification>(
+      onNotification: (ScrollMetricsNotification notification) {
+        _notifyListeners(notification);
+        return false;
+      },
+      child: _ScrollMetricsNotificationObserverScope(
+        scrollMetricsNotificationObserverState: this,
+        child: widget.child,
+      ),
+    );
+  }
+
+  @override
+  void dispose() {
+    assert(_debugAssertNotDisposed());
+    _listeners = null;
+    super.dispose();
+  }
+}

packages/flutter/test/material/scaffold_test.dart

@@ -2302,9 +2302,9 @@ void main() {
       '     PhysicalModel\n'
       '     AnimatedPhysicalModel\n'
       '     Material\n'
-      '     _ScrollNotificationObserverScope\n'
-      '     NotificationListener<ScrollNotification>\n'
-      '     ScrollNotificationObserver\n'
+      '     _ScrollMetricsNotificationObserverScope\n'
+      '     NotificationListener<ScrollMetricsNotification>\n'
+      '     ScrollMetricsNotificationObserver\n'
       '     _ScaffoldScope\n'
       '     Scaffold\n'
       '     MediaQuery\n'