[CupertinoActionSheet] Fix overflow of the overscroll section when th…

…e user scrolls far (flutter#149542)

When I implemented flutter#149334, there was a bug that if the actions section is overscrolled for too far (typically due to a fling), the overscroll background might be so long that it overflows the actions section. This PR fixes this bug.

packages/flutter/lib/src/cupertino/dialog.dart

@@ -964,8 +964,16 @@ class _ActionSheetMainSheetState extends State<_ActionSheetMainSheet> {
   bool _onScrollUpdate(ScrollUpdateNotification notification) {
     final ScrollMetrics metrics = notification.metrics;
     setState(() {
-      _topOverscroll = math.max(metrics.minScrollExtent - metrics.pixels, 0);
-      _bottomOverscroll = math.max(metrics.pixels - metrics.maxScrollExtent, 0);
+      // The sizes of the overscroll should not be longer than the height of the
+      // actions section.
+      _topOverscroll = math.min(
+        math.max(metrics.minScrollExtent - metrics.pixels, 0),
+        metrics.viewportDimension,
+      );
+      _bottomOverscroll = math.min(
+        math.max(metrics.pixels - metrics.maxScrollExtent, 0),
+        metrics.viewportDimension,
+      );
     });
     return false;
   }

packages/flutter/test/cupertino/action_sheet_test.dart

@@ -489,6 +489,48 @@ void main() {
     await gesture.up();
   });
 
+  testWidgets('Actions section correctly renders overscrolls with very far scrolls', (WidgetTester tester) async {
+    // When the scroll is really far, the overscroll might be longer than the
+    // actions section, causing overflow if not controlled.
+    final ScrollController actionScrollController = ScrollController();
+    addTearDown(actionScrollController.dispose);
+    await tester.pumpWidget(
+      createAppWithButtonThatLaunchesActionSheet(
+        Builder(builder: (BuildContext context) {
+          return CupertinoActionSheet(
+            message: Text('message' * 300),
+            actions: List<Widget>.generate(4, (int i) =>
+              CupertinoActionSheetAction(
+                onPressed: () {},
+                child: Text('Button $i'),
+              ),
+            ),
+          );
+        }),
+      ),
+    );
+
+    await tester.tap(find.text('Go'));
+    await tester.pumpAndSettle();
+
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.text('Button 0')));
+    await tester.pumpAndSettle();
+    await gesture.moveBy(const Offset(0, 40)); // A short drag to start the gesture.
+    await tester.pumpAndSettle();
+    // The drag is far enough to make the overscroll longer than the section.
+    await gesture.moveBy(const Offset(0, 1000));
+    await tester.pump();
+    // The buttons should be out of the screen
+    expect(
+      tester.getTopLeft(find.text('Button 0')).dy,
+      greaterThan(tester.getBottomLeft(find.byType(CupertinoActionSheet)).dy)
+    );
+    await expectLater(
+      find.byType(CupertinoActionSheet),
+      matchesGoldenFile('cupertinoActionSheet.long-overscroll.0.png'),
+    );
+  });
+
   testWidgets('Tap on button calls onPressed', (WidgetTester tester) async {
     bool wasPressed = false;
     await tester.pumpWidget(