Fix `StretchingOverscrollIndicator` not handling directional changes correctly (#116548)

* Fix overscroll behaviour of `StretchingOverscrollIndicator` on directional change while scrolling

* Remove trailing spaces

* Change naming of `_StretchDirection` values to `trailing` and `leading`

* Remove trailing space

* Apply suggestions from code review

* Fix stretching overscroll indicator direction on fling

* Fix issue when changing direction while recede animation is playing

* Remove trailing space

* Add test for changing direction during recede animation

AUTHORS

@@ -102,3 +102,4 @@ Tomasz Gucio <tgucio@gmail.com>
 Jason C.H <ctrysbita@outlook.com>
 Hubert Jóźwiak <hjozwiakdx@gmail.com>
 Eli Albert <crasowas@gmail.com>
+Jan Kuß <jan@kuss.dev>

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

@@ -601,6 +601,15 @@ class _GlowingOverscrollIndicatorPainter extends CustomPainter {
   }
 }
 
+enum _StretchDirection {
+  /// The [trailing] direction indicates that the content will be stretched toward
+  /// the trailing edge.
+  trailing,
+  /// The [leading] direction indicates that the content will be stretched toward
+  /// the leading edge.
+  leading,
+}
+
 /// A Material Design visual indication that a scroll view has overscrolled.
 ///
 /// A [StretchingOverscrollIndicator] listens for [ScrollNotification]s in order
@@ -689,6 +698,9 @@ class _StretchingOverscrollIndicatorState extends State<StretchingOverscrollIndi
   late final _StretchController _stretchController = _StretchController(vsync: this);
   ScrollNotification? _lastNotification;
   OverscrollNotification? _lastOverscrollNotification;
+
+  double _totalOverscroll = 0.0;
+
   bool _accepted = true;
 
   bool _handleScrollNotification(ScrollNotification notification) {
@@ -706,9 +718,11 @@ class _StretchingOverscrollIndicatorState extends State<StretchingOverscrollIndi
 
       assert(notification.metrics.axis == widget.axis);
       if (_accepted) {
+        _totalOverscroll += notification.overscroll;
+
         if (notification.velocity != 0.0) {
           assert(notification.dragDetails == null);
-          _stretchController.absorbImpact(notification.velocity.abs());
+          _stretchController.absorbImpact(notification.velocity.abs(), _totalOverscroll);
         } else {
           assert(notification.overscroll != 0.0);
           if (notification.dragDetails != null) {
@@ -716,38 +730,40 @@ class _StretchingOverscrollIndicatorState extends State<StretchingOverscrollIndi
             // which is the furthest distance a single pointer could pull on the
             // screen. This is because more than one pointer will multiply the
             // amount of overscroll - https://github.com/flutter/flutter/issues/11884
+
             final double viewportDimension = notification.metrics.viewportDimension;
-            final double distanceForPull =
-              (notification.overscroll.abs() / viewportDimension) + _stretchController.pullDistance;
+            final double distanceForPull = _totalOverscroll.abs() / viewportDimension;
             final double clampedOverscroll = clampDouble(distanceForPull, 0, 1.0);
-            _stretchController.pull(clampedOverscroll);
+            _stretchController.pull(clampedOverscroll, _totalOverscroll);
           }
         }
       }
     } else if (notification is ScrollEndNotification || notification is ScrollUpdateNotification) {
+      // Since the overscrolling ended, we reset the total overscroll amount.
+      _totalOverscroll = 0;
       _stretchController.scrollEnd();
     }
     _lastNotification = notification;
     return false;
   }
 
-  AlignmentGeometry _getAlignmentForAxisDirection(double overscroll) {
+  AlignmentGeometry _getAlignmentForAxisDirection(_StretchDirection stretchDirection) {
     // Accounts for reversed scrollables by checking the AxisDirection
     switch (widget.axisDirection) {
       case AxisDirection.up:
-        return overscroll > 0
+        return stretchDirection == _StretchDirection.trailing
             ? AlignmentDirectional.topCenter
             : AlignmentDirectional.bottomCenter;
       case AxisDirection.right:
-        return overscroll > 0
+        return stretchDirection == _StretchDirection.trailing
             ? Alignment.centerRight
             : Alignment.centerLeft;
       case AxisDirection.down:
-        return overscroll > 0
+        return stretchDirection == _StretchDirection.trailing
             ? AlignmentDirectional.bottomCenter
             : AlignmentDirectional.topCenter;
       case AxisDirection.left:
-        return overscroll > 0
+        return stretchDirection == _StretchDirection.trailing
             ? Alignment.centerLeft
             : Alignment.centerRight;
     }
@@ -784,7 +800,7 @@ class _StretchingOverscrollIndicatorState extends State<StretchingOverscrollIndi
           }
 
           final AlignmentGeometry alignment = _getAlignmentForAxisDirection(
-            _lastOverscrollNotification?.overscroll ?? 0.0
+            _stretchController.stretchDirection,
           );
 
           final double viewportDimension = _lastOverscrollNotification?.metrics.viewportDimension ?? mainAxisSize;
@@ -836,6 +852,9 @@ class _StretchController extends ChangeNotifier {
   double get pullDistance => _pullDistance;
   double _pullDistance = 0.0;
 
+  _StretchDirection get stretchDirection => _stretchDirection;
+  _StretchDirection _stretchDirection = _StretchDirection.trailing;
+
   // Constants from Android.
   static const double _exponentialScalar = math.e / 0.33;
   static const double _stretchIntensity = 0.016;
@@ -847,7 +866,7 @@ class _StretchController extends ChangeNotifier {
   /// Handle a fling to the edge of the viewport at a particular velocity.
   ///
   /// The velocity must be positive.
-  void absorbImpact(double velocity) {
+  void absorbImpact(double velocity, double totalOverscroll) {
     assert(velocity >= 0.0);
     velocity = clampDouble(velocity, 1, 10000);
     _stretchSizeTween.begin = _stretchSize.value;
@@ -855,6 +874,7 @@ class _StretchController extends ChangeNotifier {
     _stretchController.duration = Duration(milliseconds: (velocity * 0.02).round());
     _stretchController.forward(from: 0.0);
     _state = _StretchState.absorb;
+    _stretchDirection = totalOverscroll > 0 ? _StretchDirection.trailing : _StretchDirection.leading;
   }
 
   /// Handle a user-driven overscroll.
@@ -862,8 +882,19 @@ class _StretchController extends ChangeNotifier {
   /// The `normalizedOverscroll` argument should be the absolute value of the
   /// scroll distance in logical pixels, divided by the extent of the viewport
   /// in the main axis.
-  void pull(double normalizedOverscroll) {
+  void pull(double normalizedOverscroll, double totalOverscroll) {
     assert(normalizedOverscroll >= 0.0);
+
+    final _StretchDirection newStretchDirection = totalOverscroll > 0 ? _StretchDirection.trailing : _StretchDirection.leading;
+    if (_stretchDirection != newStretchDirection && _state == _StretchState.recede) {
+      // When the stretch direction changes while we are in the recede state, we need to ignore the change.
+      // If we don't, the stretch will instantly jump to the new direction with the recede animation still playing, which causes
+      // a unwanted visual abnormality (https://github.com/flutter/flutter/pull/116548#issuecomment-1414872567).
+      // By ignoring the directional change until the recede state is finished, we can avoid this.
+      return;
+    }
+
+    _stretchDirection = newStretchDirection;
     _pullDistance = normalizedOverscroll;
     _stretchSizeTween.begin = _stretchSize.value;
     final double linearIntensity =_stretchIntensity * _pullDistance;