Fix stretch edge case (#99365)

packages/flutter/lib/src/rendering/sliver_persistent_header.dart

@@ -359,7 +359,7 @@ abstract class RenderSliverScrollingPersistentHeader extends RenderSliverPersist
   @protected
   double updateGeometry() {
     double stretchOffset = 0.0;
-    if (stretchConfiguration != null && _childPosition == 0.0) {
+    if (stretchConfiguration != null) {
       stretchOffset += constraints.overlap.abs();
     }
     final double maxExtent = this.maxExtent;
@@ -596,7 +596,7 @@ abstract class RenderSliverFloatingPersistentHeader extends RenderSliverPersiste
   @protected
   double updateGeometry() {
     double stretchOffset = 0.0;
-    if (stretchConfiguration != null && _childPosition == 0.0) {
+    if (stretchConfiguration != null) {
       stretchOffset += constraints.overlap.abs();
     }
     final double maxExtent = this.maxExtent;

packages/flutter/test/widgets/slivers_appbar_stretch_test.dart

@@ -43,6 +43,105 @@ void main() {
       expect(header.child!.size.height, equals(200.0));
     });
 
+    testWidgets('fills overscroll after reverse direction input - scrolling header', (WidgetTester tester) async {
+      const Key anchor = Key('drag');
+      await tester.pumpWidget(
+        MaterialApp(
+          home: CustomScrollView(
+            physics: const BouncingScrollPhysics(),
+            slivers: <Widget>[
+              const SliverAppBar(
+                title: Text('Test'),
+                stretch: true,
+                expandedHeight: 100.0,
+              ),
+              SliverToBoxAdapter(
+                child: Container(
+                  key: anchor,
+                  height: 800,
+                ),
+              ),
+              SliverToBoxAdapter(
+                child: Container(
+                  height: 800,
+                ),
+              ),
+            ],
+          ),
+        ),
+      );
+
+      final RenderSliverScrollingPersistentHeader header = tester.renderObject(
+        find.byType(SliverAppBar),
+      );
+      expect(header.child!.size.height, equals(100.0));
+      expect(tester.getCenter(find.text('Test')).dy, 28.0);
+      // First scroll the header away
+      final TestGesture gesture = await tester.startGesture(tester.getCenter(find.byKey(anchor)));
+      await gesture.moveBy(const Offset(0.0, -100.0));
+      await tester.pump(const Duration(milliseconds: 10));
+      expect(header.child!.size.height, equals(56.0));
+      expect(tester.getCenter(find.text('Test', skipOffstage: false)).dy, -28.0);
+      // With the same gesture, scroll back and into overscroll
+      await gesture.moveBy(const Offset(0.0, 200.0));
+      await tester.pump(const Duration(milliseconds: 10));
+      // Header should stretch in overscroll
+      expect(header.child!.size.height, equals(200.0));
+      expect(tester.getCenter(find.text('Test')).dy, 28.0);
+      await gesture.up();
+      await tester.pumpAndSettle();
+    });
+
+    testWidgets('fills overscroll after reverse direction input - floating header', (WidgetTester tester) async {
+      const Key anchor = Key('drag');
+      await tester.pumpWidget(
+        MaterialApp(
+          home: CustomScrollView(
+            physics: const BouncingScrollPhysics(),
+            slivers: <Widget>[
+              const SliverAppBar(
+                title: Text('Test'),
+                stretch: true,
+                floating: true,
+                expandedHeight: 100.0,
+              ),
+              SliverToBoxAdapter(
+                child: Container(
+                  key: anchor,
+                  height: 800,
+                ),
+              ),
+              SliverToBoxAdapter(
+                child: Container(
+                  height: 800,
+                ),
+              ),
+            ],
+          ),
+        ),
+      );
+
+      final RenderSliverFloatingPersistentHeader header = tester.renderObject(
+        find.byType(SliverAppBar),
+      );
+      expect(header.child!.size.height, equals(100.0));
+      expect(tester.getCenter(find.text('Test')).dy, 28.0);
+      // First scroll the header away
+      final TestGesture gesture = await tester.startGesture(tester.getCenter(find.byKey(anchor)));
+      await gesture.moveBy(const Offset(0.0, -100.0));
+      await tester.pump(const Duration(milliseconds: 10));
+      expect(header.child!.size.height, equals(56.0));
+      expect(tester.getCenter(find.text('Test', skipOffstage: false)).dy, -28.0);
+      // With the same gesture, scroll back and into overscroll
+      await gesture.moveBy(const Offset(0.0, 200.0));
+      await tester.pump(const Duration(milliseconds: 10));
+      // Header should stretch in overscroll
+      expect(header.child!.size.height, equals(200.0));
+      expect(tester.getCenter(find.text('Test')).dy, 28.0);
+      await gesture.up();
+      await tester.pumpAndSettle();
+    });
+
     testWidgets('does not stretch without overscroll physics', (WidgetTester tester) async {
       const Key anchor = Key('drag');
       await tester.pumpWidget(