Change SliverGeometry.maxScrollObstructionExtent for RenderSliverFloatingPersistentHeader to match docs (#39263)

RenderSliverFloatingPersistentHeader does not pin to the leading edge of the viewport so the value of maxScrollObstructionExtent in it's SliverGeometry should be 0 per the docs at https://api.flutter.dev/flutter/rendering/SliverGeometry/maxScrollObstructionExtent.html.

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

@@ -415,7 +415,6 @@ abstract class RenderSliverFloatingPersistentHeader extends RenderSliverPersiste
       paintExtent: paintExtent.clamp(0.0, constraints.remainingPaintExtent),
       layoutExtent: layoutExtent.clamp(0.0, constraints.remainingPaintExtent),
       maxPaintExtent: maxExtent,
-      maxScrollObstructionExtent: maxExtent,
       hasVisualOverflow: true, // Conservatively say we do have overflow to avoid complexity.
     );
     return math.min(0.0, paintExtent - childExtent);

packages/flutter/test/rendering/sliver_persistent_header_test.dart

@@ -0,0 +1,39 @@
+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/rendering.dart';
+import '../flutter_test_alternative.dart';
+
+import 'rendering_tester.dart';
+
+void main() {
+  // Regression test for https://github.com/flutter/flutter/issues/35426.
+  test('RenderSliverFloatingPersistentHeader maxScrollObstructionExtent is 0', () {
+    final TestRenderSliverFloatingPersistentHeader header = TestRenderSliverFloatingPersistentHeader(child: RenderSizedBox(const Size(400.0, 100.0)));
+    final RenderViewport root = RenderViewport(
+      axisDirection: AxisDirection.down,
+      crossAxisDirection: AxisDirection.right,
+      offset: ViewportOffset.zero(),
+      cacheExtent: 0,
+      children: <RenderSliver>[
+        header,
+      ],
+    );
+    layout(root);
+
+    expect(header.geometry.maxScrollObstructionExtent, 0);
+  });
+}
+
+class TestRenderSliverFloatingPersistentHeader extends RenderSliverFloatingPersistentHeader {
+  TestRenderSliverFloatingPersistentHeader({
+    RenderBox child,
+  }) : super(child: child);
+
+  @override
+  double get maxExtent => 200;
+
+  @override
+  double get minExtent => 100;
+}