Reland "Ensure the engineLayer is disposed when an OpacityLayer is disabled"" (#96295)

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

@@ -1763,11 +1763,16 @@ class OpacityLayer extends OffsetLayer {
   @override
   void addToScene(ui.SceneBuilder builder) {
     assert(alpha != null);
-    bool enabled = firstChild != null;  // don't add this layer if there's no child
+
+    // Don't add this layer if there's no child.
+    bool enabled = firstChild != null;
     if (!enabled) {
+      // Ensure the engineLayer is disposed.
+      engineLayer = null;
       // TODO(dnfield): Remove this if/when we can fix https://github.com/flutter/flutter/issues/90004
       return;
     }
+
     assert(() {
       enabled = enabled && !debugDisableOpacityLayers;
       return true;

packages/flutter/test/rendering/layers_test.dart

@@ -637,6 +637,21 @@ void main() {
     expect(builder.addedPicture, true);
     expect(layer.engineLayer, isA<FakeOpacityEngineLayer>());
   });
+
+  test('OpacityLayer dispose its engineLayer if there are no children', () {
+    final OpacityLayer layer = OpacityLayer(alpha: 128);
+    final FakeSceneBuilder builder = FakeSceneBuilder();
+    layer.addToScene(builder);
+    expect(layer.engineLayer, null);
+
+    layer.append(PictureLayer(Rect.largest)..picture = FakePicture());
+    layer.addToScene(builder);
+    expect(layer.engineLayer, isA<FakeOpacityEngineLayer>());
+
+    layer.removeAllChildren();
+    layer.addToScene(builder);
+    expect(layer.engineLayer, null);
+  });
 }
 
 class FakeEngineLayer extends Fake implements EngineLayer {