Revert "Re-land reverse case for AppBar scrolled under" (#102580)

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

@@ -767,34 +767,18 @@ class _AppBarState extends State<AppBar> {
   }
 
   void _handleScrollNotification(ScrollNotification notification) {
+    if (notification is ScrollUpdateNotification) {
       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;
-      }
-    }
-
+      _scrolledUnder = notification.depth == 0
+          && notification.metrics.extentBefore > 0
+          && notification.metrics.axis == Axis.vertical;
       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

@@ -9,7 +9,6 @@ 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].
 ///
@@ -152,18 +151,7 @@ class ScrollNotificationObserverState extends State<ScrollNotificationObserver>
 
   @override
   Widget build(BuildContext context) {
-    // 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>(
+    return NotificationListener<ScrollNotification>(
       onNotification: (ScrollNotification notification) {
         _notifyListeners(notification);
         return false;
@@ -172,7 +160,6 @@ class ScrollNotificationObserverState extends State<ScrollNotificationObserver>
         scrollNotificationObserverState: this,
         child: widget.child,
       ),
-      ),
     );
   }
 
@@ -183,10 +170,3 @@ 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

@@ -349,7 +349,6 @@ 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

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