Set SliverResizingHeader's maxScrollObstructionExtent to minExtent (#162955)

The maxScrollObstructionExtent of a Sliver denotes the "maximum extent
by which this sliver can reduce the area in which content can scroll if
the sliver were pinned at the edge.".
See also the rest of the documentation of this field.

In the case of SliverResizingHeader, like with SliverPersistentHeader,
we expect this value to be minExtent.

A use case is provided in the referenced issue.

Fixes #162661.

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above.
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---------

Co-authored-by: Victor Sanni <victorsanniay@gmail.com>

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

@@ -223,7 +223,7 @@ class _RenderSliverResizingHeader extends RenderSliver
       paintExtent: math.min(childExtent, remainingPaintExtent),
       layoutExtent: clampDouble(layoutExtent, 0, remainingPaintExtent),
       maxPaintExtent: childExtent,
-      maxScrollObstructionExtent: childExtent,
+      maxScrollObstructionExtent: minExtent,
       cacheExtent: calculateCacheOffset(constraints, from: 0.0, to: childExtent),
       hasVisualOverflow: true, // Conservatively say we do have overflow to avoid complexity.
     );

packages/flutter/test/widgets/sliver_resizing_header_test.dart

@@ -357,4 +357,57 @@ void main() {
     await tester.pumpAndSettle();
     expect(getHeaderHeight(), 200);
   });
+
+  testWidgets('SliverResizingHeader maxScrollObstructionExtent', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: NestedScrollView(
+            headerSliverBuilder:
+                (BuildContext context, _) => <Widget>[
+                  SliverOverlapAbsorber(
+                    handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
+                    sliver: const SliverResizingHeader(
+                      minExtentPrototype: SizedBox(height: 100),
+                      maxExtentPrototype: SizedBox(height: 300),
+                      child: SizedBox.expand(child: Text('header')),
+                    ),
+                  ),
+                ],
+            body: Builder(
+              builder:
+                  (BuildContext context) => CustomScrollView(
+                    slivers: <Widget>[
+                      SliverOverlapInjector(
+                        handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
+                      ),
+                      SliverList(
+                        delegate: SliverChildBuilderDelegate(
+                          (BuildContext context, int index) =>
+                              SizedBox(height: 50, child: Text('$index')),
+                          childCount: 100,
+                        ),
+                      ),
+                    ],
+                  ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    double getHeaderHeight() => tester.getSize(find.text('header')).height;
+
+    expect(getHeaderHeight(), 300);
+
+    // After scrolling down 150px, the header height becomes 150px
+    await tester.drag(find.byType(NestedScrollView), const Offset(0, -150));
+    await tester.pumpAndSettle();
+    expect(getHeaderHeight(), 150);
+
+    // After scrolling down an additional 150px, the header height becomes 100px
+    await tester.drag(find.byType(NestedScrollView), const Offset(0, -150));
+    await tester.pumpAndSettle();
+    expect(getHeaderHeight(), 100);
+  });
 }