Optimize SliverMainAxisGroup/SliverCrossAxisGroup paint function (#129310)

This PR changes the paint functions for SliverMainAxisGroup and SliverCrossAxisGroup so that only visible slivers are painted.

Fixes https://github.com/flutter/flutter/issues/129214.

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

@@ -129,8 +129,10 @@ class RenderSliverCrossAxisGroup extends RenderSliver with ContainerRenderObject
     RenderSliver? child = firstChild;
 
     while (child != null) {
-      final SliverPhysicalParentData childParentData = child.parentData! as SliverPhysicalParentData;
-      context.paintChild(child, offset + childParentData.paintOffset);
+      if (child.geometry!.visible) {
+        final SliverPhysicalParentData childParentData = child.parentData! as SliverPhysicalParentData;
+        context.paintChild(child, offset + childParentData.paintOffset);
+      }
       child = childAfter(child);
     }
   }
@@ -294,8 +296,10 @@ class RenderSliverMainAxisGroup extends RenderSliver with ContainerRenderObjectM
     RenderSliver? child = lastChild;
 
     while (child != null) {
-      final SliverPhysicalParentData childParentData = child.parentData! as SliverPhysicalParentData;
-      context.paintChild(child, offset + childParentData.paintOffset);
+      if (child.geometry!.visible) {
+        final SliverPhysicalParentData childParentData = child.parentData! as SliverPhysicalParentData;
+        context.paintChild(child, offset + childParentData.paintOffset);
+      }
       child = childBefore(child);
     }
   }

packages/flutter/test/rendering/sliver_utils.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Test sliver which always attempts to paint itself whether it is visible or not.
+// Use for checking if slivers which take sliver children paints optimally.
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+
+class RenderMockSliverToBoxAdapter extends RenderSliverToBoxAdapter {
+  RenderMockSliverToBoxAdapter({
+    super.child,
+    required this.incrementCounter,
+  });
+  final void Function() incrementCounter;
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    incrementCounter();
+  }
+}
+
+class MockSliverToBoxAdapter extends SingleChildRenderObjectWidget {
+  /// Creates a sliver that contains a single box widget.
+  const MockSliverToBoxAdapter({
+    super.key,
+    super.child,
+    required this.incrementCounter,
+  });
+
+  final void Function() incrementCounter;
+
+  @override
+  RenderMockSliverToBoxAdapter createRenderObject(BuildContext context) =>
+    RenderMockSliverToBoxAdapter(incrementCounter: incrementCounter);
+}

packages/flutter/test/widgets/sliver_cross_axis_group_test.dart

@@ -6,6 +6,9 @@ import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../rendering/sliver_utils.dart';
+
+
 const double VIEWPORT_HEIGHT = 600;
 const double VIEWPORT_WIDTH = 300;
 
@@ -806,8 +809,63 @@ void main() {
     // If renderHeader._lastStartedScrollDirection is not ScrollDirection.forward, then we shouldn't see the header at all.
     expect((renderHeader.parentData! as SliverPhysicalParentData).paintOffset.dy, equals(0.0));
   });
+
+  testWidgets('SliverCrossAxisGroup skips painting invisible children', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+
+    int counter = 0;
+    void incrementCounter() {
+      counter += 1;
+    }
+
+    await tester.pumpWidget(
+      _buildSliverCrossAxisGroup(
+        controller: controller,
+        slivers: <Widget>[
+          MockSliverToBoxAdapter(
+            incrementCounter: incrementCounter,
+            child: Container(
+              height: 1000,
+              decoration: const BoxDecoration(color: Colors.amber),
+            ),
+          ),
+          MockSliverToBoxAdapter(
+            incrementCounter: incrementCounter,
+            child: Container(
+              height: 400,
+              decoration: const BoxDecoration(color: Colors.amber)
+            ),
+          ),
+          MockSliverToBoxAdapter(
+            incrementCounter: incrementCounter,
+            child: Container(
+              height: 500,
+              decoration: const BoxDecoration(color: Colors.amber)
+            ),
+          ),
+          MockSliverToBoxAdapter(
+            incrementCounter: incrementCounter,
+            child: Container(
+              height: 300,
+              decoration: const BoxDecoration(color: Colors.amber)
+            ),
+          ),
+        ],
+      ),
+    );
+    expect(counter, equals(4));
+
+    // Reset paint counter.
+    counter = 0;
+    controller.jumpTo(400);
+    await tester.pumpAndSettle();
+
+    expect(controller.offset, 400);
+    expect(counter, equals(2));
+  });
 }
 
+
 Widget _buildSliverList({
   double itemMainAxisExtent = 100,
   List<int> items = const <int>[],

packages/flutter/test/widgets/sliver_main_axis_group_test.dart

@@ -6,6 +6,9 @@ import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../rendering/sliver_utils.dart';
+
+
 const double VIEWPORT_HEIGHT = 600;
 const double VIEWPORT_WIDTH = 300;
 
@@ -604,6 +607,63 @@ void main() {
     expect(renderHeader.geometry!.paintExtent, equals(60.0));
     expect((renderHeader.parentData! as SliverPhysicalParentData).paintOffset.dy, equals(-50.0));
   });
+
+  testWidgets('SliverMainAxisGroup skips painting invisible children', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+
+    int counter = 0;
+    void incrementCounter() {
+      counter += 1;
+    }
+
+    await tester.pumpWidget(
+      _buildSliverMainAxisGroup(
+        controller: controller,
+        slivers: <Widget>[
+          MockSliverToBoxAdapter(
+            incrementCounter: incrementCounter,
+            child: Container(
+              height: 1000,
+              decoration: const BoxDecoration(color: Colors.amber),
+            ),
+          ),
+          MockSliverToBoxAdapter(
+            incrementCounter: incrementCounter,
+            child: Container(
+              height: 400,
+              decoration: const BoxDecoration(color: Colors.amber)
+            ),
+          ),
+          MockSliverToBoxAdapter(
+            incrementCounter: incrementCounter,
+            child: Container(
+              height: 500,
+              decoration: const BoxDecoration(color: Colors.amber)
+            ),
+          ),
+          MockSliverToBoxAdapter(
+            incrementCounter: incrementCounter,
+            child: Container(
+              height: 300,
+              decoration: const BoxDecoration(color: Colors.amber)
+            ),
+          ),
+        ],
+      ),
+    );
+
+    // Can only see top sliver.
+    expect(counter, equals(1));
+
+    // Reset paint counter.
+    counter = 0;
+    controller.jumpTo(1000);
+    await tester.pumpAndSettle();
+
+    // Can only see second and third slivers.
+    expect(controller.offset, 1000);
+    expect(counter, equals(2));
+  });
 }
 
 Widget _buildSliverList({