[bugfix] SliverGeometry is not valid: The "layoutExtent" is negative (#48947)

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

@@ -1445,7 +1445,7 @@ class RenderSliverOverlapAbsorber extends RenderSliver with RenderObjectWithChil
       scrollExtent: childLayoutGeometry.scrollExtent - childLayoutGeometry.maxScrollObstructionExtent,
       paintExtent: childLayoutGeometry.paintExtent,
       paintOrigin: childLayoutGeometry.paintOrigin,
-      layoutExtent: childLayoutGeometry.paintExtent - childLayoutGeometry.maxScrollObstructionExtent,
+      layoutExtent: math.max(0, childLayoutGeometry.paintExtent - childLayoutGeometry.maxScrollObstructionExtent),
       maxPaintExtent: childLayoutGeometry.maxPaintExtent,
       maxScrollObstructionExtent: childLayoutGeometry.maxScrollObstructionExtent,
       hitTestExtent: childLayoutGeometry.hitTestExtent,

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -676,6 +676,12 @@ void main() {
     await gesture.up();
     debugDefaultTargetPlatformOverride = null;
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/39963.
+  testWidgets('NestedScrollView with SliverOverlapAbsorber in or out of the first screen', (WidgetTester tester) async {
+    await tester.pumpWidget(const _TestLayoutExtentIsNegative(1));
+    await tester.pumpWidget(const _TestLayoutExtentIsNegative(10));
+  });
 }
 
 class TestHeader extends SliverPersistentHeaderDelegate {
@@ -692,3 +698,60 @@ class TestHeader extends SliverPersistentHeaderDelegate {
   @override
   bool shouldRebuild(TestHeader oldDelegate) => false;
 }
+
+class _TestLayoutExtentIsNegative extends StatelessWidget {
+  const _TestLayoutExtentIsNegative(this.widgetCountBeforeSliverOverlapAbsorber);
+  final int widgetCountBeforeSliverOverlapAbsorber;
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        appBar: AppBar(
+          title: const Text('Test'),
+        ),
+        body: NestedScrollView(
+          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
+            return <Widget>[
+              ...List<Widget>.generate(widgetCountBeforeSliverOverlapAbsorber, (_) {
+                return SliverToBoxAdapter(
+                  child: Container(
+                    color: Colors.red,
+                    height: 200,
+                    margin:const EdgeInsets.all(20),
+                  ),
+                );
+              },),
+              SliverOverlapAbsorber(
+                handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
+                sliver: SliverAppBar(
+                  pinned: true,
+                  forceElevated: innerBoxIsScrolled,
+                  backgroundColor: Colors.blue[300],
+                  title: Container(
+                    height: 50,
+                    child: const Center(
+                      child: Text('Sticky Header'),
+                    ),
+                  ),
+                ),
+              )
+            ];
+          },
+          body: Container(
+            height: 2000,
+            margin: const EdgeInsets.only(top: 50),
+            child: ListView(
+              children: List<Widget>.generate(3, (_) {
+                return Container(
+                  color: Colors.green[200],
+                  height: 200,
+                  margin: const EdgeInsets.all(20),
+                );
+              },),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}