Fix race condition causing crash when interacting with an animating scrollable (#164392)

Fix https://github.com/flutter/flutter/issues/163297
Fix https://github.com/flutter/flutter/issues/14452

Partial patch: https://github.com/flutter/flutter/pull/37267

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

@@ -872,7 +872,10 @@ class ScrollableState extends State<Scrollable>
     assert(_drag == null);
     _drag = position.drag(details, _disposeDrag);
     assert(_drag != null);
-    assert(_hold == null);
+    // _hold might be non-null if the scroll position is currently animating.
+    if (_hold != null) {
+      _disposeHold();
+    }
   }
 
   void _handleDragUpdate(DragUpdateDetails details) {

packages/flutter/test/widgets/scrollable_animations_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/gestures.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -103,6 +104,58 @@ void main() {
       reason: 'Expected an animation.',
     );
   });
+
+  testWidgets('HoldActivity can interrupt ScrollPosition.animateTo', (WidgetTester tester) async {
+    const double animationExtent = 100.0;
+    const double dragExtent = 30.0;
+    final ScrollController controller = ScrollController();
+    addTearDown(controller.dispose);
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: NotificationListener<ScrollNotification>(
+          onNotification: (ScrollNotification notification) {
+            controller.position.animateTo(
+              animationExtent,
+              duration: const Duration(seconds: 1),
+              curve: Curves.linear,
+            );
+            return true;
+          },
+          child: ListView(
+            controller: controller,
+            dragStartBehavior: DragStartBehavior.down,
+            children: List<Widget>.generate(
+              80,
+              (int i) => Text('$i', textDirection: TextDirection.ltr),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expectNoAnimation();
+
+    // Drag to initiate the scroll animation.
+    await tester.drag(find.byType(Scrollable), const Offset(0.0, 1.0));
+    await tester.pump();
+
+    // Pump to halfway through the animation.
+    await tester.pump(const Duration(milliseconds: 500));
+    expect(controller.position.pixels, animationExtent / 2);
+
+    // Interrupt the scroll animation.
+    final TestGesture gesture = await tester.startGesture(
+      tester.getCenter(find.byType(Scrollable)),
+    );
+    await gesture.moveBy(const Offset(0.0, dragExtent));
+    await gesture.up();
+
+    await tester.pump(const Duration(milliseconds: 500));
+
+    // The drag stops the animation, and the drag extent is respected.
+    expect(controller.position.pixels, (animationExtent / 2) - dragExtent);
+  });
 }
 
 void expectNoAnimation() {