Improve Scrollbar drag behavior (flutter#112434)

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

@@ -686,6 +686,17 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
     return scrollableExtent * thumbOffsetLocal / thumbMovableExtent;
   }
 
+  /// The thumb's corresponding scroll offset in the track.
+  double getThumbScrollOffset() {
+    final double scrollableExtent = _lastMetrics!.maxScrollExtent - _lastMetrics!.minScrollExtent;
+
+    final double fractionPast = (scrollableExtent > 0)
+      ? clampDouble(_lastMetrics!.pixels / scrollableExtent, 0.0, 1.0)
+      : 0;
+
+    return fractionPast * (_traversableTrackExtent - _thumbExtent);
+  }
+
   // Converts between a scroll position and the corresponding position in the
   // thumb track.
   double _getScrollToTrack(ScrollMetrics metrics, double thumbExtent) {
@@ -1446,15 +1457,15 @@ class RawScrollbar extends StatefulWidget {
 /// Provides defaults gestures for dragging the scrollbar thumb and tapping on the
 /// scrollbar track.
 class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProviderStateMixin<T> {
-  Offset? _dragScrollbarAxisOffset;
+  Offset? _startDragScrollbarAxisOffset;
+  double? _startDragThumbOffset;
   ScrollController? _currentController;
   Timer? _fadeoutTimer;
   late AnimationController _fadeoutAnimationController;
   late Animation<double> _fadeoutOpacityAnimation;
   final GlobalKey  _scrollbarPainterKey = GlobalKey();
   bool _hoverIsActive = false;
 
-
   /// Used to paint the scrollbar.
   ///
   /// Can be customized by subclasses to change scrollbar behavior by overriding
@@ -1688,30 +1699,30 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
 
   void _updateScrollPosition(Offset updatedOffset) {
     assert(_currentController != null);
-    assert(_dragScrollbarAxisOffset != null);
+    assert(_startDragScrollbarAxisOffset != null);
+    assert(_startDragThumbOffset != null);
 
     final ScrollPosition position = _currentController!.position;
     late double primaryDelta;
     switch (position.axisDirection) {
       case AxisDirection.up:
-        primaryDelta = _dragScrollbarAxisOffset!.dy - updatedOffset.dy;
+        primaryDelta = _startDragScrollbarAxisOffset!.dy - updatedOffset.dy;
         break;
       case AxisDirection.right:
-        primaryDelta = updatedOffset.dx -_dragScrollbarAxisOffset!.dx;
+        primaryDelta = updatedOffset.dx -_startDragScrollbarAxisOffset!.dx;
         break;
       case AxisDirection.down:
-        primaryDelta = updatedOffset.dy -_dragScrollbarAxisOffset!.dy;
+        primaryDelta = updatedOffset.dy -_startDragScrollbarAxisOffset!.dy;
         break;
       case AxisDirection.left:
-        primaryDelta = _dragScrollbarAxisOffset!.dx - updatedOffset.dx;
+        primaryDelta = _startDragScrollbarAxisOffset!.dx - updatedOffset.dx;
         break;
     }
 
     // Convert primaryDelta, the amount that the scrollbar moved since the last
-    // time _updateScrollPosition was called, into the coordinate space of the scroll
+    // time when drag started, into the coordinate space of the scroll
     // position, and jump to that position.
-    final double scrollOffsetLocal = scrollbarPainter.getTrackToScroll(primaryDelta);
-    final double scrollOffsetGlobal = scrollOffsetLocal + position.pixels;
+    final double scrollOffsetGlobal = scrollbarPainter.getTrackToScroll(primaryDelta + _startDragThumbOffset!);
     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);
@@ -1784,7 +1795,8 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     }
     _fadeoutTimer?.cancel();
     _fadeoutAnimationController.forward();
-    _dragScrollbarAxisOffset = localPosition;
+    _startDragScrollbarAxisOffset = localPosition;
+    _startDragThumbOffset = scrollbarPainter.getThumbScrollOffset();
   }
 
   /// Handler called when a currently active long press gesture moves.
@@ -1803,7 +1815,6 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
       return;
     }
     _updateScrollPosition(localPosition);
-    _dragScrollbarAxisOffset = localPosition;
   }
 
   /// Handler called when a long press has ended.
@@ -1816,7 +1827,8 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
       return;
     }
     _maybeStartFadeoutTimer();
-    _dragScrollbarAxisOffset = null;
+    _startDragScrollbarAxisOffset = null;
+    _startDragThumbOffset = null;
     _currentController = null;
   }
 
@@ -1958,7 +1970,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
         scrollbarPainter.update(metrics, metrics.axisDirection);
       }
     } else if (notification is ScrollEndNotification) {
-      if (_dragScrollbarAxisOffset == null) {
+      if (_startDragScrollbarAxisOffset == null) {
         _maybeStartFadeoutTimer();
       }
     }

packages/flutter/test/widgets/scrollbar_test.dart

@@ -2717,4 +2717,75 @@ void main() {
 
     expect(scrollController.offset, 0.0);
   });
+
+  testWidgets('The thumb should follow the pointer when the scroll metrics changed during dragging', (WidgetTester tester) async {
+    // Regressing test for https://github.com/flutter/flutter/issues/112072
+    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,
+                // cacheExtent: double.maxFinite,
+                slivers: <Widget>[
+                  SliverList(
+                    delegate: SliverChildBuilderDelegate(
+                      (BuildContext context, int index) {
+                        final double height;
+                        if (index < 10) {
+                          height = 100;
+                        } else {
+                          height = 500;
+                        }
+                        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();
+
+    await dragScrollbarGesture.moveBy(const Offset(0.0, scrollAmount));
+    await tester.pumpAndSettle();
+
+    await dragScrollbarGesture.up();
+    await tester.pumpAndSettle();
+
+    // The view has scrolled more than it would have by a swipe gesture of the
+    // same distance.
+    expect(scrollController.offset, greaterThan((100.0 * 10 + 500.0 * 90) / 3));
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 200.0, 800.0, 218.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+  });
 }