fix a scroll bar stutter bug (#121786)

Fix a scrolling stutter caused by dragging scrollbar

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

@@ -1676,29 +1676,41 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   void _updateScrollPosition(Offset updatedOffset) {
     assert(_cachedController != null);
     assert(_startDragScrollbarAxisOffset != null);
+    assert(_lastDragUpdateOffset != null);
     assert(_startDragThumbOffset != null);
 
     final ScrollPosition position = _cachedController!.position;
-    late double primaryDelta;
+    late double primaryDeltaFromDragStart;
+    late double primaryDeltaFromLastDragUpdate;
     switch (position.axisDirection) {
       case AxisDirection.up:
-        primaryDelta = _startDragScrollbarAxisOffset!.dy - updatedOffset.dy;
+        primaryDeltaFromDragStart = _startDragScrollbarAxisOffset!.dy - updatedOffset.dy;
+        primaryDeltaFromLastDragUpdate = _lastDragUpdateOffset!.dy - updatedOffset.dy;
         break;
       case AxisDirection.right:
-        primaryDelta = updatedOffset.dx -_startDragScrollbarAxisOffset!.dx;
+        primaryDeltaFromDragStart = updatedOffset.dx -_startDragScrollbarAxisOffset!.dx;
+        primaryDeltaFromLastDragUpdate = updatedOffset.dx -_lastDragUpdateOffset!.dx;
         break;
       case AxisDirection.down:
-        primaryDelta = updatedOffset.dy -_startDragScrollbarAxisOffset!.dy;
+        primaryDeltaFromDragStart = updatedOffset.dy -_startDragScrollbarAxisOffset!.dy;
+        primaryDeltaFromLastDragUpdate = updatedOffset.dy -_lastDragUpdateOffset!.dy;
         break;
       case AxisDirection.left:
-        primaryDelta = _startDragScrollbarAxisOffset!.dx - updatedOffset.dx;
+        primaryDeltaFromDragStart = _startDragScrollbarAxisOffset!.dx - updatedOffset.dx;
+        primaryDeltaFromLastDragUpdate = _lastDragUpdateOffset!.dx - updatedOffset.dx;
         break;
     }
 
     // Convert primaryDelta, the amount that the scrollbar moved since the last
-    // time when drag started, into the coordinate space of the scroll
+    // time when drag started or last updated, into the coordinate space of the scroll
     // position, and jump to that position.
-    final double scrollOffsetGlobal = scrollbarPainter.getTrackToScroll(primaryDelta + _startDragThumbOffset!);
+    double scrollOffsetGlobal = scrollbarPainter.getTrackToScroll(primaryDeltaFromDragStart + _startDragThumbOffset!);
+    if (primaryDeltaFromDragStart > 0 && scrollOffsetGlobal < position.pixels
+        || primaryDeltaFromDragStart < 0 && scrollOffsetGlobal > position.pixels) {
+      // Adjust the position value if the scrolling direction conflicts with
+      // the dragging direction due to scroll metrics shrink.
+      scrollOffsetGlobal = position.pixels + scrollbarPainter.getTrackToScroll(primaryDeltaFromLastDragUpdate);
+    }
     if (scrollOffsetGlobal != position.pixels) {
       // Ensure we don't drag into overscroll if the physics do not allow it.
       final double physicsAdjustment = position.physics.applyBoundaryConditions(position, scrollOffsetGlobal);
@@ -1772,6 +1784,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     _fadeoutTimer?.cancel();
     _fadeoutAnimationController.forward();
     _startDragScrollbarAxisOffset = localPosition;
+    _lastDragUpdateOffset = localPosition;
     _startDragThumbOffset = scrollbarPainter.getThumbScrollOffset();
     _thumbDragging = true;
   }
@@ -1786,7 +1799,6 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     if (_lastDragUpdateOffset == localPosition) {
       return;
     }
-    _lastDragUpdateOffset = localPosition;
     final ScrollPosition position = _cachedController!.position;
     if (!position.physics.shouldAcceptUserOffset(position)) {
       return;
@@ -1796,6 +1808,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
       return;
     }
     _updateScrollPosition(localPosition);
+    _lastDragUpdateOffset = localPosition;
   }
 
   /// Handler called when a long press has ended.

packages/flutter/test/widgets/scrollbar_test.dart

@@ -2740,7 +2740,6 @@ void main() {
               controller: scrollController,
               child: CustomScrollView(
                 controller: scrollController,
-                // cacheExtent: double.maxFinite,
                 slivers: <Widget>[
                   SliverList(
                     delegate: SliverChildBuilderDelegate(
@@ -2796,6 +2795,70 @@ void main() {
     );
   });
 
+  testWidgets('The scrollable should not stutter when the scroll metrics shrink during dragging', (WidgetTester tester) async {
+    // Regressing test for https://github.com/flutter/flutter/issues/121574
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: RawScrollbar(
+              isAlwaysShown: true,
+              controller: scrollController,
+              child: CustomScrollView(
+                controller: scrollController,
+                slivers: <Widget>[
+                  SliverList(
+                    delegate: SliverChildBuilderDelegate(
+                          (BuildContext context, int index) {
+                        final double height;
+                        if (index < 10) {
+                          height = 500;
+                        } else {
+                          height = 100;
+                        }
+                        return SizedBox(
+                          height: height,
+                          child: Text('$index'),
+                        );
+                      },
+                      childCount: 100,
+                    ),
+                  ),
+                ],
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+
+    // Drag the thumb down to scroll down.
+    const double scrollAmount = 100;
+    final TestGesture dragScrollbarGesture = await tester.startGesture(const Offset(797.0, 5.0));
+    await tester.pumpAndSettle();
+    await dragScrollbarGesture.moveBy(const Offset(0.0, scrollAmount));
+    await tester.pumpAndSettle();
+
+    final double lastPosition = scrollController.offset;
+    // The view has scrolled more than it would have by a swipe gesture of the
+    // same distance.
+    expect(lastPosition, greaterThan((100.0 * 10 + 500.0 * 90) / 6));
+
+    await dragScrollbarGesture.moveBy(const Offset(0.0, scrollAmount));
+    await tester.pumpAndSettle();
+
+    await dragScrollbarGesture.up();
+    await tester.pumpAndSettle();
+
+    expect(scrollController.offset, greaterThan(lastPosition));
+  });
+
   testWidgets('The bar support mouse wheel event', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/pull/109659
     final ScrollController scrollController = ScrollController();