Re-land reverse case for AppBar scrolled under (#102343)

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

@@ -767,18 +767,34 @@ class _AppBarState extends State<AppBar> {
   }
 
   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;
+    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) {
     return MaterialStateProperty.resolveAs<Color?>(widgetColor, states)

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].
 ///
@@ -151,7 +152,18 @@ class ScrollNotificationObserverState extends State<ScrollNotificationObserver>
 
   @override
   Widget build(BuildContext context) {
-    return NotificationListener<ScrollNotification>(
+    // A ScrollMetricsNotification allows listeners to be notified for an
+    // initial state, as well as if the content dimensions change without
+    // scrolling.
+    return NotificationListener<ScrollMetricsNotification>(
+      onNotification: (ScrollMetricsNotification notification) {
+        _notifyListeners(_ConvertedScrollMetricsNotification(
+            metrics: notification.metrics,
+            context: notification.context,
+        ));
+        return false;
+      },
+      child: NotificationListener<ScrollNotification>(
         onNotification: (ScrollNotification notification) {
           _notifyListeners(notification);
           return false;
@@ -160,6 +172,7 @@ class ScrollNotificationObserverState extends State<ScrollNotificationObserver>
           scrollNotificationObserverState: this,
           child: widget.child,
         ),
+      ),
     );
   }
 
@@ -170,3 +183,10 @@ class ScrollNotificationObserverState extends State<ScrollNotificationObserver>
     super.dispose();
   }
 }
+
+class _ConvertedScrollMetricsNotification extends ScrollNotification {
+  _ConvertedScrollMetricsNotification({
+    required super.metrics,
+    required super.context,
+  });
+}

packages/flutter/test/material/debug_test.dart

@@ -349,6 +349,7 @@ void main() {
       '     Material\n'
       '     _ScrollNotificationObserverScope\n'
       '     NotificationListener<ScrollNotification>\n'
+      '     NotificationListener<ScrollMetricsNotification>\n'
       '     ScrollNotificationObserver\n'
       '     _ScaffoldScope\n'
       '     Scaffold-[LabeledGlobalKey<ScaffoldState>#00000]\n'

packages/flutter/test/material/scaffold_test.dart

@@ -2339,6 +2339,7 @@ void main() {
       '     Material\n'
       '     _ScrollNotificationObserverScope\n'
       '     NotificationListener<ScrollNotification>\n'
+      '     NotificationListener<ScrollMetricsNotification>\n'
       '     ScrollNotificationObserver\n'
       '     _ScaffoldScope\n'
       '     Scaffold\n'