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

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

@@ -276,14 +276,21 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   Rect? _trackRect;
   late double _thumbOffset;
 
-  /// Update with new [ScrollMetrics]. The scrollbar will show and redraw itself
-  /// based on these new metrics.
+  /// Update with new [ScrollMetrics]. If the metrics change, 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();
@@ -930,16 +937,17 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    _maybeTriggerScrollbar();
+    _maybeRequestEmptyScrollEvent();
   }
 
   // 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() {
+  void _maybeRequestEmptyScrollEvent() {
+    if (!showScrollbar)
+      return;
     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
@@ -1013,7 +1021,6 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
         return true;
       }());
       scrollController!.position.didUpdateScrollPositionBy(0);
-      }
     });
   }
 
@@ -1035,15 +1042,16 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @override
   void didUpdateWidget(T oldWidget) {
     super.didUpdateWidget(oldWidget);
-    if (widget.isAlwaysShown != oldWidget.isAlwaysShown) {
+    // 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 == true) {
-        _maybeTriggerScrollbar();
+      _maybeRequestEmptyScrollEvent();
       _fadeoutAnimationController.animateTo(1.0);
-      } else {
+    } else if (widget.isAlwaysShown != oldWidget.isAlwaysShown) {
       _fadeoutAnimationController.reverse();
     }
   }
-  }
 
   void _updateScrollPosition(double primaryDelta) {
     assert(_currentController != null);
@@ -1203,13 +1211,19 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
       return false;
 
     final ScrollMetrics metrics = notification.metrics;
-    if (metrics.maxScrollExtent <= metrics.minScrollExtent)
+    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();
       return false;
+    }
 
     if (notification is ScrollUpdateNotification ||
       notification is OverscrollNotification) {
       // Any movements always makes the scrollbar start showing up.
-      if (_fadeoutAnimationController.status != AnimationStatus.forward)
+      if (_fadeoutAnimationController.status != AnimationStatus.forward
+          && _fadeoutAnimationController.status != AnimationStatus.completed)
         _fadeoutAnimationController.forward();
 
       _fadeoutTimer?.cancel();

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1152,4 +1152,35 @@ 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.
+  });
 }