update the scrollbar (#82687)

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

@@ -334,16 +334,29 @@ 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;
+
+    final ScrollMetrics? oldMetrics = _lastMetrics;
     _lastMetrics = metrics;
     _lastAxisDirection = axisDirection;
+
+    bool _needPaint(ScrollMetrics? metrics) => metrics != null && metrics.maxScrollExtent > metrics.minScrollExtent;
+    if (!_needPaint(oldMetrics) && !_needPaint(metrics))
+      return;
+
     notifyListeners();
   }
 
@@ -526,7 +539,8 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   void paint(Canvas canvas, Size size) {
     if (_lastAxisDirection == null
         || _lastMetrics == null
-        || fadeoutOpacityAnimation.value == 0.0)
+        || fadeoutOpacityAnimation.value == 0.0
+        || _lastMetrics!.maxScrollExtent <= _lastMetrics!.minScrollExtent)
       return;
 
     // Skip painting if there's not enough space.
@@ -1519,18 +1533,37 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     );
   }
 
+  bool _handleScrollMetricsNotification(ScrollMetricsNotification notification) {
+    if (!widget.notificationPredicate(ScrollUpdateNotification(metrics: notification.metrics, context: notification.context)))
+      return false;
+    if (showScrollbar) {
+      if (_fadeoutAnimationController.status != AnimationStatus.forward
+          && _fadeoutAnimationController.status != AnimationStatus.completed)
+        _fadeoutAnimationController.forward();
+    }
+    scrollbarPainter.update(notification.metrics, notification.metrics.axisDirection);
+    return false;
+  }
+
   bool _handleScrollNotification(ScrollNotification notification) {
     if (!widget.notificationPredicate(notification))
       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();
+      scrollbarPainter.update(metrics, metrics.axisDirection);
       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();
@@ -1658,12 +1691,13 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     super.dispose();
   }
 
-
   @override
   Widget build(BuildContext context) {
     updateScrollbarPainter();
 
-    return NotificationListener<ScrollNotification>(
+    return NotificationListener<ScrollMetricsNotification>(
+      onNotification: _handleScrollMetricsNotification,
+      child: NotificationListener<ScrollNotification>(
         onNotification: _handleScrollNotification,
         child: RepaintBoundary(
           child: RawGestureDetector(
@@ -1703,6 +1737,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
             ),
           ),
         ),
+      ),
     );
   }
 }

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1565,4 +1565,76 @@ void main() {
           ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 8.0)));
   });
 
+  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.
+  });
+
+  testWidgets('The bar can show or hide when the window size change', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    Widget buildFrame() {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: RawScrollbar(
+              isAlwaysShown: true,
+              controller: scrollController,
+              child: const SingleChildScrollView(
+                child: SizedBox(
+                  width: double.infinity,
+                  height: 600.0,
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+    tester.binding.window.physicalSizeTestValue = const Size(800.0, 600.0);
+    tester.binding.window.devicePixelRatioTestValue = 1;
+    addTearDown(tester.binding.window.clearPhysicalSizeTestValue);
+    addTearDown(tester.binding.window.clearDevicePixelRatioTestValue);
+
+    await tester.pumpWidget(buildFrame());
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+    expect(find.byType(RawScrollbar), isNot(paints..rect())); // Not shown.
+
+    tester.binding.window.physicalSizeTestValue = const Size(800.0, 599.0);
+    await tester.pumpAndSettle();
+    expect(find.byType(RawScrollbar), paints..rect()..rect()); // Show the bar.
+
+    tester.binding.window.physicalSizeTestValue = const Size(800.0, 600.0);
+    await tester.pumpAndSettle();
+    expect(find.byType(RawScrollbar), isNot(paints..rect())); // Not shown.
+  });
 }