Test case for overlap

main.dart

import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets('SliverClipRect updates clip when overlap changes even if geometry is same', (WidgetTester tester) async {
    final ScrollController controller = ScrollController();
    await tester.pumpWidget(
      Directionality(
        textDirection: TextDirection.ltr,
        child: CustomScrollView(
          controller: controller,
          slivers: <Widget>[
            const SliverPersistentHeader(
              delegate: MyDelegate(maxExtent: 100, minExtent: 100),
              pinned: true,
            ),
            const SliverToBoxAdapter(child: SizedBox(height: 100)), // Spacer
            SliverClipRect(
              sliver: SliverToBoxAdapter(
                child: Container(
                  height: 100,
                  color: const Color(0xFF2196F3),
                ),
              ),
            ),
            const SliverToBoxAdapter(
              child: SizedBox(height: 1000),
            ),
          ],
        ),
      ),
    );

    final RenderSliverClipRect renderSliver = tester.renderObject(find.byType(SliverClipRect));

    // Initial state: no overlap
    // Header 0..100, Spacer 100..200, ClipRect 200..300.
    expect(renderSliver.constraints.overlap, 0.0);
    expect(renderSliver.getClip()!.top, 0.0);

    // Scroll by 150.
    // Spacer is scrolled off. ClipRect starts at y=50.
    // Since Header is pinned at 0..100, it overlaps ClipRect by 50px.
    controller.jumpTo(150);
    await tester.pump();

    expect(renderSliver.constraints.overlap, 50.0);
    // This should fail if the bug exists because the ClipRect is still fully visible (paintExtent=100),
    // so its geometry didn't change, and _clip was not nulled.
    expect(renderSliver.getClip()!.top, 50.0);
  });
}

class MyDelegate extends SliverPersistentHeaderDelegate {
  const MyDelegate({required this.maxExtent, required this.minExtent});
  @override
  final double maxExtent;
  @override
  final double minExtent;
  @override
  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) => SizedBox(height: maxExtent);
  @override
  bool shouldRebuild(MyDelegate oldDelegate) => maxExtent != oldDelegate.maxExtent || minExtent != oldDelegate.minExtent;
}