Do not block vertical drag gestures in CupertinoSheetRoute body (#161696)

Fixes #161623.

Changes how the VerticalGestureRecognizer watches the sheet body. Before
it stacked over the sheet and won all gesture arenas with vertical
drags, so it didn't allow scrolling in the sheet. Now it has the sheet
content as its child, so it will defer to child vertical gestures. This
PR will allow scrolling to work in the sheet, but further work needs to
be done to improve scrolling and drag to dismiss working together, see
#161687.



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Author: MitchellGoodwin

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

@@ -669,12 +669,10 @@ class _CupertinoDownGestureDetectorState<T> extends State<_CupertinoDownGestureD
 
   @override
   Widget build(BuildContext context) {
-    return Stack(
-      fit: StackFit.passthrough,
-      children: <Widget>[
-        widget.child,
-        Listener(onPointerDown: _handlePointerDown, behavior: HitTestBehavior.translucent),
-      ],
+    return Listener(
+      onPointerDown: _handlePointerDown,
+      behavior: HitTestBehavior.translucent,
+      child: widget.child,
     );
   }
 }

packages/flutter/test/cupertino/sheet_test.dart

@@ -844,5 +844,72 @@ void main() {
 
       expect(find.text('Page 1'), findsOneWidget);
     });
+
+    testWidgets('Sheet should not block nested scroll', (WidgetTester tester) async {
+      final GlobalKey homeKey = GlobalKey();
+
+      Widget sheetScaffoldContent(BuildContext context) {
+        return ListView(
+          children: const <Widget>[
+            Text('Top of Scroll'),
+            SizedBox(width: double.infinity, height: 100),
+            Text('Middle of Scroll'),
+            SizedBox(width: double.infinity, height: 100),
+          ],
+        );
+      }
+
+      await tester.pumpWidget(
+        CupertinoApp(
+          home: CupertinoPageScaffold(
+            key: homeKey,
+            child: Center(
+              child: Column(
+                children: <Widget>[
+                  const Text('Page 1'),
+                  CupertinoButton(
+                    onPressed: () {
+                      showCupertinoSheet<void>(
+                        context: homeKey.currentContext!,
+                        pageBuilder: (BuildContext context) {
+                          return CupertinoPageScaffold(child: sheetScaffoldContent(context));
+                        },
+                      );
+                    },
+                    child: const Text('Push Page 2'),
+                  ),
+                ],
+              ),
+            ),
+          ),
+        ),
+      );
+
+      await tester.tap(find.text('Push Page 2'));
+      await tester.pumpAndSettle();
+
+      expect(find.text('Top of Scroll'), findsOneWidget);
+      final double startPosition = tester.getTopLeft(find.text('Middle of Scroll')).dy;
+
+      final TestGesture gesture = await tester.createGesture();
+
+      await gesture.down(const Offset(100, 100));
+
+      // Need 2 events to form a valid drag.
+      await tester.pump(const Duration(milliseconds: 100));
+      await gesture.moveTo(const Offset(100, 80), timeStamp: const Duration(milliseconds: 100));
+      await tester.pump(const Duration(milliseconds: 200));
+      await gesture.moveTo(const Offset(100, 50), timeStamp: const Duration(milliseconds: 200));
+
+      await tester.pumpAndSettle();
+
+      final double endPosition = tester.getTopLeft(find.text('Middle of Scroll')).dy;
+
+      // Final position should be higher.
+      expect(endPosition, lessThan(startPosition));
+
+      await gesture.up();
+      await tester.pumpAndSettle();
+    });
   });
 }