Fix the compositing of placeholder canvas backgrounds

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 101 matching lines @@
 
 static inline bool isAcceleratedCanvas(const LayoutObject& layoutObject) {
   if (layoutObject.isCanvas()) {
     HTMLCanvasElement* canvas = toHTMLCanvasElement(layoutObject.node());
     if (CanvasRenderingContext* context = canvas->renderingContext())
       return context->isAccelerated();
   }
   return false;
 }
 
+static inline bool isPlaceholderCanvas(const LayoutObject& layoutObject) {

[xlai (Olivia), 2017/03/24 20:24:59]

You don't need to add this function. There is "isCanvasControlledByOffscreen"
function that's exactly same as this.

+  if (layoutObject.isCanvas()) {
+    HTMLCanvasElement* canvas = toHTMLCanvasElement(layoutObject.node());
+    return canvas->surfaceLayerBridge();
+  }
+  return false;
+}
+
 static inline bool isCanvasControlledByOffscreen(
     const LayoutObject& layoutObject) {
   if (layoutObject.isCanvas()) {
     HTMLCanvasElement* canvas = toHTMLCanvasElement(layoutObject.node());
     if (canvas->surfaceLayerBridge())
       return true;
   }
   return false;
 }
 
@@ skipping 288 matching lines @@
         m_graphicsLayer->setContentsOpaque(
             !Color(layer->backgroundColor()).hasAlpha());
       }
     } else {
       m_graphicsLayer->setContentsOpaque(false);
     }
   } else if (m_backgroundLayer) {
     m_graphicsLayer->setContentsOpaque(false);
     m_backgroundLayer->setContentsOpaque(
         m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
+  } else if (isPlaceholderCanvas(layoutObject())) {
+    // TODO(crbug.com/705019): Contents could be opaque, but that cannot be
+    // determined from the main thread. Or can it?
+    m_graphicsLayer->setContentsOpaque(false);
   } else {
     // For non-root layers, background is painted by the scrolling contents
     // layer if all backgrounds are background attachment local, otherwise
     // background is painted by the primary graphics layer.
     if (hasScrollingLayer() && m_backgroundPaintsOntoScrollingContentsLayer) {
       // Backgrounds painted onto the foreground are clipped by the padding box
       // rect.
       // TODO(flackr): This should actually check the entire overflow rect
       // within the scrolling contents layer but since we currently only trigger
       // this for solid color backgrounds the answer will be the same.
@@ skipping 3058 matching lines @@
   } else if (graphicsLayer == m_decorationOutlineLayer.get()) {
     name = "Decoration Layer";
   } else {
     ASSERT_NOT_REACHED();
   }
 
   return name;
 }
 
 }  // namespace blink