update ScrollMetricsNotification (#87421)

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

@@ -558,7 +558,7 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
       // It isn't safe to trigger the ScrollMetricsNotification if we are in
       // the middle of rendering the frame, the developer is likely to schedule
       // a new frame(build scheduled during frame is illegal).
-      if (_lastMetrics != null && !_haveScheduledUpdateNotification) {
+      if (!_haveScheduledUpdateNotification) {
         scheduleMicrotask(didUpdateScrollMetrics);
         _haveScheduledUpdateNotification = true;
       }

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

@@ -1224,25 +1224,16 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    _maybeTriggerScrollbar();
+    assert(_debugScheduleCheckHasValidScrollPosition());
   }
 
-  // Waits one frame and cause an empty scroll event (zero delta pixels).
-  //
-  // This allows the thumb to show immediately when isAlwaysShown is true.
-  // A scroll event is required in order to paint the thumb.
-  void _maybeTriggerScrollbar() {
+  bool _debugScheduleCheckHasValidScrollPosition() {
+    if (!showScrollbar)
+      return true;
     WidgetsBinding.instance!.addPostFrameCallback((Duration duration) {
-      final ScrollController? scrollController = widget.controller ?? PrimaryScrollController.of(context);
-      if (showScrollbar) {
-        _fadeoutTimer?.cancel();
-        // Wait one frame and cause an empty scroll event.  This allows the
-        // thumb to show immediately when isAlwaysShown is true. A scroll
-        // event is required in order to paint the thumb.
       assert(_debugCheckHasValidScrollPosition());
-        scrollController!.position.didUpdateScrollPositionBy(0);
-      }
     });
+    return true;
   }
 
   void _validateInteractions(AnimationStatus status) {
@@ -1365,7 +1356,8 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     super.didUpdateWidget(oldWidget);
     if (widget.isAlwaysShown != oldWidget.isAlwaysShown) {
       if (widget.isAlwaysShown == true) {
-        _maybeTriggerScrollbar();
+        assert(_debugScheduleCheckHasValidScrollPosition());
+        _fadeoutTimer?.cancel();
         _fadeoutAnimationController.animateTo(1.0);
       } else {
         _fadeoutAnimationController.reverse();

packages/flutter/test/cupertino/scrollbar_test.dart

@@ -5,6 +5,7 @@
 import 'dart:ui' as ui;
 
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -389,9 +390,16 @@ void main() {
         );
       }
 
+      final FlutterExceptionHandler? handler = FlutterError.onError;
+      FlutterErrorDetails? error;
+      FlutterError.onError = (FlutterErrorDetails details) {
+        error = details;
+      };
+
       await tester.pumpWidget(viewWithScroll());
-      final AssertionError exception = tester.takeException() as AssertionError;
-      expect(exception, isAssertionError);
+      expect(error, isNotNull);
+
+      FlutterError.onError = handler;
     },
   );
 

packages/flutter/test/widgets/scroll_notification_test.dart

@@ -21,8 +21,9 @@ void main() {
       );
     }
     await tester.pumpWidget(buildFrame(1200.0));
-    expect(events.length, 0);
+    expect(events.length, 1);
 
+    events.clear();
     await tester.pumpWidget(buildFrame(1000.0));
     // Change the content dimensions will trigger a new event.
     expect(events.length, 1);

packages/flutter/test/widgets/scrollbar_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:ui' as ui;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/src/physics/utils.dart' show nearEqual;
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -1143,6 +1144,12 @@ void main() {
   });
 
   testWidgets('RawScrollbar.isAlwaysShown asserts that a ScrollPosition is attached', (WidgetTester tester) async {
+    final FlutterExceptionHandler? handler = FlutterError.onError;
+    FlutterErrorDetails? error;
+    FlutterError.onError = (FlutterErrorDetails details) {
+      error = details;
+    };
+
     await tester.pumpWidget(
       Directionality(
         textDirection: TextDirection.ltr,
@@ -1163,12 +1170,15 @@ void main() {
       ),
     );
     await tester.pumpAndSettle();
-    final AssertionError exception = tester.takeException() as AssertionError;
-    expect(exception, isAssertionError);
+
+    expect(error, isNotNull);
+    final AssertionError exception = error!.exception as AssertionError;
     expect(
       exception.message,
       contains("The Scrollbar's ScrollController has no ScrollPosition attached."),
     );
+
+    FlutterError.onError = handler;
   });
 
   testWidgets('Interactive scrollbars should have a valid scroll controller', (WidgetTester tester) async {