Bug 574621. Make sure top level content documents get an opaque background. (patch modified to review comments by romaxa) r=tnikkel

layout/base/nsPresShell.cpp

@@ -5786,17 +5786,22 @@ void PresShell::UpdateCanvasBackground()
   // If we have a frame tree and it has style information that
   // specifies the background color of the canvas, update our local
   // cache of that color.
-  nsIFrame* rootFrame = FrameConstructor()->GetRootElementStyleFrame();
-  if (rootFrame) {
+  nsIFrame* rootStyleFrame = FrameConstructor()->GetRootElementStyleFrame();
+  if (rootStyleFrame) {
     nsStyleContext* bgStyle =
-      nsCSSRendering::FindRootFrameBackground(rootFrame);
+      nsCSSRendering::FindRootFrameBackground(rootStyleFrame);
     // XXX We should really be passing the canvasframe, not the root element
     // style frame but we don't have access to the canvasframe here. It isn't
     // a problem because only a few frames can return something other than true
     // and none of them would be a canvas frame or root element style frame.
     mCanvasBackgroundColor =
       nsCSSRendering::DetermineBackgroundColor(GetPresContext(), bgStyle,
-                                               rootFrame);
+                                               rootStyleFrame);
+    if (nsLayoutUtils::GetCrossDocParentFrame(FrameManager()->GetRootFrame()) &&
+        !nsContentUtils::IsChildOfSameType(mDocument)) {
+      mCanvasBackgroundColor =
+        NS_ComposeColors(mPresContext->DefaultBackgroundColor(), mCanvasBackgroundColor);
+    }
   }
 
   // If the root element of the document (ie html) has style 'display: none'