Revert "improve the scrollbar behavior when viewport size changed (#76102)" (#80059)

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

@@ -276,21 +276,14 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   Rect? _trackRect;
   late double _thumbOffset;
 
-  /// Update with new [ScrollMetrics]. If the metrics change, the scrollbar will
-  /// show and redraw itself based on these new metrics.
+  /// Update with new [ScrollMetrics]. The scrollbar will show and redraw itself
+  /// based on these new metrics.
   ///
   /// The scrollbar will remain on screen.
   void update(
     ScrollMetrics metrics,
     AxisDirection axisDirection,
   ) {
-    if (_lastMetrics != null &&
-        _lastMetrics!.extentBefore == metrics.extentBefore &&
-        _lastMetrics!.extentInside == metrics.extentInside &&
-        _lastMetrics!.extentAfter == metrics.extentAfter &&
-        _lastAxisDirection == axisDirection)
-      return;
-
     _lastMetrics = metrics;
     _lastAxisDirection = axisDirection;
     notifyListeners();
@@ -937,17 +930,16 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    _maybeRequestEmptyScrollEvent();
+    _maybeTriggerScrollbar();
   }
 
   // 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 _maybeRequestEmptyScrollEvent() {
-    if (!showScrollbar)
-      return;
+  void _maybeTriggerScrollbar() {
     WidgetsBinding.instance!.addPostFrameCallback((Duration duration) {
+      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
@@ -1021,6 +1013,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
           return true;
         }());
         scrollController!.position.didUpdateScrollPositionBy(0);
+      }
     });
   }
 
@@ -1042,16 +1035,15 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @override
   void didUpdateWidget(T oldWidget) {
     super.didUpdateWidget(oldWidget);
-    // If `isAlwaysShown` is true and does not change,
-    // it may be necessary to trigger a scroll event to show or hide the bar when the
-    // scrollable widget viewport size changed.
+    if (widget.isAlwaysShown != oldWidget.isAlwaysShown) {
       if (widget.isAlwaysShown == true) {
-      _maybeRequestEmptyScrollEvent();
+        _maybeTriggerScrollbar();
         _fadeoutAnimationController.animateTo(1.0);
-    } else if (widget.isAlwaysShown != oldWidget.isAlwaysShown) {
+      } else {
         _fadeoutAnimationController.reverse();
       }
     }
+  }
 
   void _updateScrollPosition(double primaryDelta) {
     assert(_currentController != null);
@@ -1211,19 +1203,13 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
       return false;
 
     final ScrollMetrics metrics = notification.metrics;
-    if (metrics.maxScrollExtent <= metrics.minScrollExtent) {
-      // Hide the bar when the Scrollable widget has no space to scroll.
-      if (_fadeoutAnimationController.status != AnimationStatus.dismissed
-          && _fadeoutAnimationController.status != AnimationStatus.reverse)
-        _fadeoutAnimationController.reverse();
+    if (metrics.maxScrollExtent <= metrics.minScrollExtent)
       return false;
-    }
 
     if (notification is ScrollUpdateNotification ||
       notification is OverscrollNotification) {
       // Any movements always makes the scrollbar start showing up.
-      if (_fadeoutAnimationController.status != AnimationStatus.forward
-          && _fadeoutAnimationController.status != AnimationStatus.completed)
+      if (_fadeoutAnimationController.status != AnimationStatus.forward)
         _fadeoutAnimationController.forward();
 
       _fadeoutTimer?.cancel();

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1152,35 +1152,4 @@ void main() {
         ),
     );
   });
-
-  testWidgets('The bar can show or hide when the viewport size change', (WidgetTester tester) async {
-    final ScrollController scrollController = ScrollController();
-    Widget buildFrame(double height) {
-      return Directionality(
-        textDirection: TextDirection.ltr,
-        child: MediaQuery(
-          data: const MediaQueryData(),
-          child: RawScrollbar(
-            controller: scrollController,
-            isAlwaysShown: true,
-            child: SingleChildScrollView(
-                controller: scrollController,
-                child: SizedBox(width: double.infinity, height: height)
-            ),
-          ),
-        ),
-      );
-    }
-    await tester.pumpWidget(buildFrame(600.0));
-    await tester.pumpAndSettle();
-    expect(find.byType(RawScrollbar), isNot(paints..rect())); // Not shown.
-
-    await tester.pumpWidget(buildFrame(600.1));
-    await tester.pumpAndSettle();
-    expect(find.byType(RawScrollbar), paints..rect()..rect()); // Show the bar.
-
-    await tester.pumpWidget(buildFrame(600.0));
-    await tester.pumpAndSettle();
-    expect(find.byType(RawScrollbar), isNot(paints..rect())); // Hide the bar.
-  });
 }