remove forced compositing from opacity (#105334)

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

@@ -13,6 +13,7 @@ import 'package:flutter/services.dart';
 import 'package:vector_math/vector_math_64.dart';
 
 import 'box.dart';
+import 'debug.dart';
 import 'layer.dart';
 import 'layout_helper.dart';
 import 'object.dart';
@@ -884,16 +885,6 @@ class RenderOpacity extends RenderProxyBox {
        _alpha = ui.Color.getAlphaFromOpacity(opacity),
        super(child);
 
-  @override
-  bool get alwaysNeedsCompositing => child != null && (_alpha > 0);
-
-  @override
-  OffsetLayer updateCompositedLayer({required covariant OpacityLayer? oldLayer}) {
-    final OpacityLayer updatedLayer = oldLayer ?? OpacityLayer();
-    updatedLayer.alpha = _alpha;
-    return updatedLayer;
-  }
-
   int _alpha;
 
   /// The fraction to scale the child's alpha value.
@@ -914,13 +905,9 @@ class RenderOpacity extends RenderProxyBox {
     if (_opacity == value) {
       return;
     }
-    final bool didNeedCompositing = alwaysNeedsCompositing;
     final bool wasVisible = _alpha != 0;
     _opacity = value;
     _alpha = ui.Color.getAlphaFromOpacity(_opacity);
-    if (didNeedCompositing != alwaysNeedsCompositing) {
-      markNeedsCompositingBitsUpdate();
-    }
     markNeedsPaint();
     if (wasVisible != (_alpha != 0) && !alwaysIncludeSemantics) {
       markNeedsSemanticsUpdate();
@@ -950,19 +937,40 @@ class RenderOpacity extends RenderProxyBox {
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (child != null) {
+    if (child == null) {
+      return;
+    }
     if (_alpha == 0) {
       // No need to keep the layer. We'll create a new one if necessary.
       layer = null;
       return;
     }
-      assert(needsCompositing);
+    if (_alpha == 255) {
+      layer = null;
+      return super.paint(context, offset);
+    }
+    // Due to https://github.com/flutter/flutter/issues/48417 this will always need to be
+    // composited on the web.
+    if (needsCompositing || kIsWeb) {
       layer = context.pushOpacity(offset, _alpha, super.paint, oldLayer: layer as OpacityLayer?);
       assert(() {
-        layer!.debugCreator = debugCreator;
+        layer?.debugCreator = debugCreator;
         return true;
       }());
+      return;
     }
+
+    // debugDisableOpacityLayers is used by the SceneBuilder to remove opacity layers, but
+    // if the framework is not asked to composite will also need to remove the opacity here.
+    if (kDebugMode && debugDisableOpacityLayers) {
+      super.paint(context, offset);
+      return;
+    }
+    final Color color = Color.fromRGBO(0, 0, 0, opacity);
+    final Canvas canvas = context.canvas;
+    canvas.saveLayer(size != null ? offset & size : null, Paint()..color = color);
+    super.paint(context, offset);
+    canvas.restore();
   }
 
   @override

packages/flutter/test/rendering/proxy_box_test.dart

@@ -197,7 +197,7 @@ void main() {
 
     expect(data.lengthInBytes, equals(20 * 20 * 4));
     expect(data.elementSizeInBytes, equals(1));
-    expect(getPixel(0, 0), equals(0x00000080));
+    expect(getPixel(0, 0), equals(0x0000007F));
     expect(getPixel(image.width - 1, 0 ), equals(0xffffffff));
 
     final OffsetLayer layer = boundary.debugLayer! as OffsetLayer;
@@ -206,7 +206,7 @@ void main() {
     expect(image.width, equals(20));
     expect(image.height, equals(20));
     data = (await image.toByteData())!;
-    expect(getPixel(0, 0), equals(0x00000080));
+    expect(getPixel(0, 0), equals(0x0000007F));
     expect(getPixel(image.width - 1, 0 ), equals(0xffffffff));
 
     // non-zero offsets.
@@ -215,7 +215,7 @@ void main() {
     expect(image.height, equals(30));
     data = (await image.toByteData())!;
     expect(getPixel(0, 0), equals(0x00000000));
-    expect(getPixel(10, 10), equals(0x00000080));
+    expect(getPixel(10, 10), equals(0x0000007F));
     expect(getPixel(image.width - 1, 0), equals(0x00000000));
     expect(getPixel(image.width - 1, 10), equals(0xffffffff));
 
@@ -225,7 +225,7 @@ void main() {
     expect(image.height, equals(60));
     data = (await image.toByteData())!;
     expect(getPixel(0, 0), equals(0x00000000));
-    expect(getPixel(20, 20), equals(0x00000080));
+    expect(getPixel(20, 20), equals(0x0000007F));
     expect(getPixel(image.width - 1, 0), equals(0x00000000));
     expect(getPixel(image.width - 1, 20), equals(0xffffffff));
   }, skip: isBrowser); // https://github.com/flutter/flutter/issues/49857
@@ -240,13 +240,13 @@ void main() {
     expect(renderOpacity.needsCompositing, false);
   });
 
-  test('RenderOpacity does composite if it is opaque', () {
+  test('RenderOpacity does not composite if it is opaque', () {
     final RenderOpacity renderOpacity = RenderOpacity(
       child: RenderSizedBox(const Size(1.0, 1.0)), // size doesn't matter
     );
 
     layout(renderOpacity, phase: EnginePhase.composite);
-    expect(renderOpacity.needsCompositing, true);
+    expect(renderOpacity.needsCompositing, false);
   });
 
   test('RenderOpacity reuses its layer', () {

packages/flutter/test/widgets/debug_test.dart

@@ -285,8 +285,8 @@ void main() {
                   child: Placeholder(),
                 ),
                 const Opacity(
-                  opacity: 1.0,
-                  child: Placeholder(),
+                  opacity: 0.9, // ensure compositing is used.
+                  child: RepaintBoundary(child: Placeholder()),
                 ),
                 ImageFiltered(
                   imageFilter: ImageFilter.blur(sigmaX: 10.0, sigmaY: 10.0),