Fix the compositing of placeholder canvas backgrounds

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index a67d5daa6b6ed92a786f29761a9469fa43a62f88..7e8313263f100fd842ed9ea4014e7d6c941d8a10 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -119,6 +119,14 @@ static inline bool isAcceleratedCanvas(const LayoutObject& layoutObject) {
   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()) {
@@ -427,6 +435,10 @@ void CompositedLayerMapping::updateContentsOpaque() {
     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