[framework] make opacity widget create a repaint boundary (#116788)

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

@@ -890,6 +890,9 @@ class RenderOpacity extends RenderProxyBox {
   @override
   bool get alwaysNeedsCompositing => child != null && _alpha > 0;
 
+  @override
+  bool get isRepaintBoundary => alwaysNeedsCompositing;
+
   int _alpha;
 
   /// The fraction to scale the child's alpha value.
@@ -917,7 +920,7 @@ class RenderOpacity extends RenderProxyBox {
     if (didNeedCompositing != alwaysNeedsCompositing) {
       markNeedsCompositingBitsUpdate();
     }
-    markNeedsPaint();
+    markNeedsCompositedLayerUpdate();
     if (wasVisible != (_alpha != 0) && !alwaysIncludeSemantics) {
       markNeedsSemanticsUpdate();
     }
@@ -944,23 +947,19 @@ class RenderOpacity extends RenderProxyBox {
     return _alpha > 0;
   }
 
+  @override
+  OffsetLayer updateCompositedLayer({required covariant OpacityLayer? oldLayer}) {
+    final OpacityLayer layer = oldLayer ?? OpacityLayer();
+    layer.alpha = _alpha;
+    return layer;
+  }
+
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (child == null) {
-      return;
-    }
-    if (_alpha == 0) {
-      // No need to keep the layer. We'll create a new one if necessary.
-      layer = null;
+    if (child == null || _alpha == 0) {
       return;
     }
-
-    assert(needsCompositing);
-    layer = context.pushOpacity(offset, _alpha, super.paint, oldLayer: layer as OpacityLayer?);
-    assert(() {
-      layer!.debugCreator = debugCreator;
-      return true;
-    }());
+    super.paint(context, offset);
   }
 
   @override

packages/flutter/test/widgets/opacity_repaint_test.dart

@@ -0,0 +1,96 @@
+// 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.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('RenderOpacity avoids repainting and does not drop layer at fully opaque', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 0.0,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 0);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 0.1,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 1,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderOpacity allows opacity layer to be dropped at 0 opacity', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 0.5,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 0.0,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+    expect(tester.layers, isNot(contains(isA<OpacityLayer>())));
+  });
+}
+
+class TestWidget extends SingleChildRenderObjectWidget {
+  const TestWidget({super.key, super.child});
+
+  @override
+  RenderObject createRenderObject(BuildContext context) {
+    return RenderTestObject();
+  }
+}
+
+class RenderTestObject extends RenderProxyBox {
+  static int paintCount = 0;
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    paintCount += 1;
+    super.paint(context, offset);
+  }
+}