The OverscrollIndicator should not overflow the scrollable view's edge (#64239)

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

@@ -253,9 +253,15 @@ class _GlowingOverscrollIndicatorState extends State<GlowingOverscrollIndicator>
     // scroll, e.g. when scrolling in the opposite direction again to hide
     // the glow. Otherwise, the glow would always stay in a fixed position,
     // even if the top of the content already scrolled away.
-    _leadingController._paintOffsetScrollPixels = -notification.metrics.pixels;
+    // For example (CustomScrollView with sliver before center), the scroll
+    // extent is [-200.0, 300.0], scroll in the opposite direction with 10.0 pixels
+    // before glow disappears, so the current pixels is -190.0,
+    // in this case, we should move the glow up 10.0 pixels and should not
+    // overflow the scrollable widget's edge. https://github.com/flutter/flutter/issues/64149.
+    _leadingController._paintOffsetScrollPixels =
+      -math.min(notification.metrics.pixels - notification.metrics.minScrollExtent, _leadingController._paintOffset);
     _trailingController._paintOffsetScrollPixels =
-        -(notification.metrics.maxScrollExtent - notification.metrics.pixels);
+      -math.min(notification.metrics.maxScrollExtent - notification.metrics.pixels, _trailingController._paintOffset);
 
     if (notification is OverscrollNotification) {
       _GlowController controller;
@@ -269,9 +275,6 @@ class _GlowingOverscrollIndicatorState extends State<GlowingOverscrollIndicator>
       final bool isLeading = controller == _leadingController;
       if (_lastNotificationType != OverscrollNotification) {
         final OverscrollIndicatorNotification confirmationNotification = OverscrollIndicatorNotification(leading: isLeading);
-        // It is possible that the scroll extent starts at non-zero.
-        if (isLeading)
-          confirmationNotification.paintOffset = notification.metrics.minScrollExtent;
         confirmationNotification.dispatch(context);
         _accepted[isLeading] = confirmationNotification._accepted;
         if (_accepted[isLeading]) {

packages/flutter/test/widgets/overscroll_indicator_test.dart

@@ -362,7 +362,116 @@ void main() {
     expect(painter, paints..save()..translate(y: 0.0)..scale()..circle());
   });
 
-  group('Modify glow position', () {
+  testWidgets('CustomScrollView overscroll indicator works well with [CustomScrollView.center] and [OverscrollIndicatorNotification.paintOffset]', (WidgetTester tester) async {
+    final Key centerKey = UniqueKey();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: ScrollConfiguration(
+          behavior: TestScrollBehavior2(),
+          child: NotificationListener<OverscrollIndicatorNotification>(
+            onNotification: (OverscrollIndicatorNotification notification) {
+              if (notification.leading) {
+                notification.paintOffset = 50.0;
+              }
+              return false;
+            },
+            child: CustomScrollView(
+              center: centerKey,
+              physics: const AlwaysScrollableScrollPhysics(),
+              slivers: <Widget>[
+                SliverList(
+                  delegate: SliverChildBuilderDelegate(
+                        (BuildContext context, int index) => Text('First sliver $index',),
+                    childCount: 2,
+                  ),
+                ),
+                SliverList(
+                  key: centerKey,
+                  delegate: SliverChildBuilderDelegate(
+                        (BuildContext context, int index) => Text('Second sliver $index',),
+                    childCount: 5,
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(find.text('First sliver 1'), findsNothing);
+
+    await slowDrag(tester, const Offset(200.0, 200.0), const Offset(0.0, 5.0)); // offset will be magnified ten times
+    expect(find.text('First sliver 1'), findsOneWidget);
+    final RenderObject painter = tester.renderObject(find.byType(CustomPaint));
+    // The OverscrollIndicator should respect the [OverscrollIndicatorNotification.paintOffset] setting.
+    expect(painter, paints..save()..translate(y: 50.0)..scale()..circle());
+  });
+
+  testWidgets('The OverscrollIndicator should not overflow the scrollable view edge', (WidgetTester tester) async {
+    // Regressing test for https://github.com/flutter/flutter/issues/64149
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: NotificationListener<OverscrollIndicatorNotification>(
+          onNotification: (OverscrollIndicatorNotification notification) {
+            notification.paintOffset = 50.0; // both the leading and trailing indicator have a 50.0 pixels offset.
+            return false;
+          },
+          child: const CustomScrollView(
+            slivers: <Widget>[
+              SliverToBoxAdapter(child: SizedBox(height: 2000.0)),
+            ],
+          ),
+        ),
+      ),
+    );
+    final RenderObject painter = tester.renderObject(find.byType(CustomPaint));
+    await slowDrag(tester, const Offset(200.0, 200.0), const Offset(0.0, 5.0));
+    expect(painter, paints..save()..translate(y: 50.0)..scale()..circle());
+    // Reverse scroll (30 pixels), and the offset < notification.paintOffset.
+    await tester.dragFrom(const Offset(200.0, 200.0), const Offset(0.0, -30.0));
+    await tester.pump();
+    // The OverscrollIndicator should move with the CustomScrollView.
+    expect(painter, paints..save()..translate(y: 50.0 - 30.0)..scale()..circle());
+
+    // Reverse scroll (30+20 pixels) and offset == notification.paintOffset.
+    await tester.dragFrom(const Offset(200.0, 200.0), const Offset(0.0, -20.0));
+    await tester.pump();
+    expect(painter, paints..save()..translate(y: 50.0 - 50.0)..scale()..circle());
+
+    // Reverse scroll (30+20+10 pixels) and offset > notification.paintOffset.
+    await tester.dragFrom(const Offset(200.0, 200.0), const Offset(0.0, -10.0));
+    await tester.pump();
+    // The OverscrollIndicator should not overflow the CustomScrollView's edge.
+    expect(painter, paints..save()..translate(y: 50.0 - 50.0)..scale()..circle());
+
+    await tester.pumpAndSettle(); // Finish the leading indicator.
+
+    // trigger the trailing indicator
+    await slowDrag(tester, const Offset(200.0, 200.0), const Offset(0.0, -200.0));
+    expect(painter, paints..scale(y: -1.0)..save()..translate(y: 50.0)..scale()..circle());
+
+    // Reverse scroll (30 pixels), and the offset < notification.paintOffset.
+    await tester.dragFrom(const Offset(200.0, 200.0), const Offset(0.0, 30.0));
+    await tester.pump();
+    // The OverscrollIndicator should move with the CustomScrollView.
+    expect(painter, paints..scale(y: -1.0)..save()..translate(y: 50.0 - 30.0)..scale()..circle());
+
+    // Reverse scroll (30+20 pixels) and offset == notification.paintOffset.
+    await tester.dragFrom(const Offset(200.0, 200.0), const Offset(0.0, 20.0));
+    await tester.pump();
+    expect(painter, paints..scale(y: -1.0)..save()..translate(y: 50.0 - 50.0)..scale()..circle());
+
+    // Reverse scroll (30+20+10 pixels) and offset > notification.paintOffset.
+    await tester.dragFrom(const Offset(200.0, 200.0), const Offset(0.0, 10.0));
+    await tester.pump();
+    // The OverscrollIndicator should not overflow the CustomScrollView's edge.
+    expect(painter, paints..scale(y: -1.0)..save()..translate(y: 50.0 - 50.0)..scale()..circle());
+  });
+
+  group('[OverscrollIndicatorNotification.paintOffset] test', () {
     testWidgets('Leading', (WidgetTester tester) async {
       await tester.pumpWidget(
         Directionality(
@@ -384,11 +493,12 @@ void main() {
       );
       final RenderObject painter = tester.renderObject(find.byType(CustomPaint));
       await slowDrag(tester, const Offset(200.0, 200.0), const Offset(0.0, 5.0));
+      // The OverscrollIndicator should respect the [OverscrollIndicatorNotification.paintOffset] setting.
       expect(painter, paints..save()..translate(y: 50.0)..scale()..circle());
       // Reverse scroll direction.
       await tester.dragFrom(const Offset(200.0, 200.0), const Offset(0.0, -30.0));
       await tester.pump();
-      // The painter should follow the scroll direction.
+      // The OverscrollIndicator should move with the CustomScrollView.
       expect(painter, paints..save()..translate(y: 50.0 - 30.0)..scale()..circle());
     });
 
@@ -415,11 +525,12 @@ void main() {
       await tester.dragFrom(const Offset(200.0, 200.0), const Offset(200.0, -10000.0));
       await tester.pump();
       await slowDrag(tester, const Offset(200.0, 200.0), const Offset(0.0, -5.0));
+      // The OverscrollIndicator should respect the [OverscrollIndicatorNotification.paintOffset] setting.
       expect(painter, paints..scale(y: -1.0)..save()..translate(y: 50.0)..scale()..circle());
       // Reverse scroll direction.
       await tester.dragFrom(const Offset(200.0, 200.0), const Offset(0.0, 30.0));
       await tester.pump();
-      // The painter should follow the scroll direction.
+      // The OverscrollIndicator should move with the CustomScrollView.
       expect(painter, paints..scale(y: -1.0)..save()..translate(y: 50.0 - 30.0)..scale()..circle());
     });
   });