[framework] attempt non-key solution (flutter#128273)

Adds a special opacity widget that does not act like a repaint boundary.

Better solution for flutter#128138

Author: Jonah Williams

packages/flutter/lib/src/material/flexible_space_bar.dart

@@ -6,6 +6,7 @@ import 'dart:math' as math;
 import 'dart:ui' as ui;
 
 import 'package:flutter/foundation.dart' show clampDouble;
+import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
@@ -251,17 +252,12 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
             left: 0.0,
             right: 0.0,
             height: height,
-            child: Opacity(
-              // We need the child widget to repaint, however both the opacity
-              // and potentially `widget.background` can be constant which won't
-              // lead to repainting.
-              // see: https://github.com/flutter/flutter/issues/127836
-              key: ValueKey<double>(topPadding),
+            child: _FlexibleSpaceHeaderOpacity(
               // IOS is relying on this semantics node to correctly traverse
               // through the app bar when it is collapsed.
               alwaysIncludeSemantics: true,
               opacity: opacity,
-              child: widget.background,
+              child: widget.background
             ),
           ));
 
@@ -426,3 +422,33 @@ class FlexibleSpaceBarSettings extends InheritedWidget {
         || isScrolledUnder != oldWidget.isScrolledUnder;
   }
 }
+
+// We need the child widget to repaint, however both the opacity
+// and potentially `widget.background` can be constant which won't
+// lead to repainting.
+// see: https://github.com/flutter/flutter/issues/127836
+class _FlexibleSpaceHeaderOpacity extends SingleChildRenderObjectWidget {
+  const _FlexibleSpaceHeaderOpacity({required this.opacity, required super.child, required this.alwaysIncludeSemantics});
+
+  final double opacity;
+  final bool alwaysIncludeSemantics;
+
+  @override
+  RenderObject createRenderObject(BuildContext context) {
+    return _RenderFlexibleSpaceHeaderOpacity(opacity: opacity, alwaysIncludeSemantics: alwaysIncludeSemantics);
+  }
+
+  @override
+  void updateRenderObject(BuildContext context, covariant _RenderFlexibleSpaceHeaderOpacity renderObject) {
+    renderObject
+      ..alwaysIncludeSemantics = alwaysIncludeSemantics
+      ..opacity = opacity;
+  }
+}
+
+class _RenderFlexibleSpaceHeaderOpacity extends RenderOpacity {
+  _RenderFlexibleSpaceHeaderOpacity({super.opacity, super.alwaysIncludeSemantics});
+
+  @override
+  bool get isRepaintBoundary => false;
+}

packages/flutter/test/material/flexible_space_bar_test.dart

@@ -8,6 +8,7 @@
 library;
 
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import '../widgets/semantics_tester.dart';
@@ -159,7 +160,10 @@ void main() {
       ),
     );
 
-    final Opacity backgroundOpacity = tester.firstWidget(find.byType(Opacity));
+    final dynamic backgroundOpacity = tester.firstWidget(
+      find.byWidgetPredicate((Widget widget) => widget.runtimeType.toString() == '_FlexibleSpaceHeaderOpacity'));
+    // accessing private type member.
+    // ignore: avoid_dynamic_calls
     expect(backgroundOpacity.opacity, 1.0);
   });
 
@@ -348,7 +352,7 @@ void main() {
                               rect: const Rect.fromLTRB(0.0, 0.0, 800.0, 56.0),
                               children: <TestSemantics>[
                                 TestSemantics(
-                                  id: 18,
+                                  id: 11,
                                   rect: const Rect.fromLTRB(0.0, 36.0, 800.0, 92.0),
                                   label: 'Expanded title',
                                   textDirection: TextDirection.ltr,
@@ -790,15 +794,15 @@ void main() {
       home: SubCategoryScreenView(),
     ));
 
-    expect(RebuildTracker.count, 1);
+    expect(RenderRebuildTracker.count, 1);
 
     // We drag up to fully collapse the space bar.
     for (int i = 0; i < 20; i++) {
       await tester.drag(find.byKey(SubCategoryScreenView.scrollKey), const Offset(0, -50.0));
       await tester.pumpAndSettle();
     }
 
-    expect(RebuildTracker.count, greaterThan(1));
+    expect(RenderRebuildTracker.count, greaterThan(1));
   });
 }
 
@@ -825,15 +829,22 @@ class TestDelegate extends SliverPersistentHeaderDelegate {
   bool shouldRebuild(TestDelegate oldDelegate) => false;
 }
 
-class RebuildTracker extends StatelessWidget {
+class RebuildTracker extends SingleChildRenderObjectWidget {
   const RebuildTracker({super.key});
 
+  @override
+  RenderObject createRenderObject(BuildContext context) {
+    return RenderRebuildTracker();
+  }
+}
+
+class RenderRebuildTracker extends RenderProxyBox {
   static int count = 0;
 
   @override
-  Widget build(BuildContext context)  {
+  void paint(PaintingContext context, Offset offset) {
     count++;
-    return const SizedBox(width: 100, height: 100);
+    super.paint(context, offset);
   }
 }