fuchsia: Remove Opacity hole-punch (#21527)

Author: arbreng

flow/layers/child_scene_layer.cc

@@ -20,26 +20,10 @@ void ChildSceneLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
 
   context->child_scene_layer_exists_below = true;
   CheckForChildLayerBelow(context);
-
-  // An alpha "hole punch" is required if the frame behind us is not opaque.
-  if (!context->is_opaque) {
-    set_paint_bounds(
-        SkRect::MakeXYWH(offset_.fX, offset_.fY, size_.fWidth, size_.fHeight));
-  }
 }
 
 void ChildSceneLayer::Paint(PaintContext& context) const {
-  TRACE_EVENT0("flutter", "ChildSceneLayer::Paint");
-  FML_DCHECK(needs_painting());
-  FML_DCHECK(needs_system_composite());
-
-  // If we are being rendered into our own frame using the system compositor,
-  // then it is neccesary to "punch a hole" in the canvas/frame behind us so
-  // that group opacity looks correct.
-  SkPaint paint;
-  paint.setColor(SK_ColorTRANSPARENT);
-  paint.setBlendMode(SkBlendMode::kSrc);
-  context.leaf_nodes_canvas->drawRect(paint_bounds(), paint);
+  FML_NOTREACHED();
 }
 
 void ChildSceneLayer::UpdateScene(SceneUpdateContext& context) {

flow/layers/layer.h

@@ -59,7 +59,6 @@ struct PrerollContext {
   // These allow us to track properties like elevation, opacity, and the
   // prescence of a platform view during Preroll.
   bool has_platform_view = false;
-  bool is_opaque = true;
 #if defined(LEGACY_FUCHSIA_EMBEDDER)
   // True if, during the traversal so far, we have seen a child_scene_layer.
   // Informs whether a layer needs to be system composited.

flow/layers/opacity_layer.cc

@@ -14,19 +14,15 @@ OpacityLayer::OpacityLayer(SkAlpha alpha, const SkPoint& offset)
 
 void OpacityLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   TRACE_EVENT0("flutter", "OpacityLayer::Preroll");
+  FML_DCHECK(!GetChildContainer()->layers().empty());  // We can't be a leaf.
 
-  ContainerLayer* container = GetChildContainer();
-  FML_DCHECK(!container->layers().empty());  // OpacityLayer can't be a leaf.
-
-  const bool parent_is_opaque = context->is_opaque;
   SkMatrix child_matrix = matrix;
   child_matrix.postTranslate(offset_.fX, offset_.fY);
 
   // Similar to what's done in TransformLayer::Preroll, we have to apply the
   // reverse transformation to the cull rect to properly cull child layers.
   context->cull_rect = context->cull_rect.makeOffset(-offset_.fX, -offset_.fY);
 
-  context->is_opaque = parent_is_opaque && (alpha_ == SK_AlphaOPAQUE);
   context->mutators_stack.PushTransform(
       SkMatrix::Translate(offset_.fX, offset_.fY));
   context->mutators_stack.PushOpacity(alpha_);
@@ -35,7 +31,6 @@ void OpacityLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   ContainerLayer::Preroll(context, child_matrix);
   context->mutators_stack.Pop();
   context->mutators_stack.Pop();
-  context->is_opaque = parent_is_opaque;
 
   {
     set_paint_bounds(paint_bounds().makeOffset(offset_.fX, offset_.fY));

flow/layers/opacity_layer_unittests.cc

@@ -20,8 +20,9 @@ TEST_F(OpacityLayerTest, LeafLayer) {
   auto layer =
       std::make_shared<OpacityLayer>(SK_AlphaOPAQUE, SkPoint::Make(0.0f, 0.0f));
 
-  EXPECT_DEATH_IF_SUPPORTED(layer->Preroll(preroll_context(), SkMatrix()),
-                            "\\!container->layers\\(\\)\\.empty\\(\\)");
+  EXPECT_DEATH_IF_SUPPORTED(
+      layer->Preroll(preroll_context(), SkMatrix()),
+      "\\!GetChildContainer\\(\\)->layers\\(\\)\\.empty\\(\\)");
 }
 
 TEST_F(OpacityLayerTest, PaintingEmptyLayerDies) {