Bug 453566 - HTML element with rgba(0,0,0,0) background produces garbage on screen. r+sr=roc

layout/base/nsCSSRendering.cpp

@@ -1299,21 +1299,24 @@ nsCSSRendering::PaintBackground(nsPresContext* aPresContext,
 
   nsIViewManager* vm = aPresContext->GetViewManager();
 
-  if (NS_GET_A(canvasColor.mBackgroundColor) == 0) {
-    nsIView* rootView;
-    vm->GetRootView(rootView);
-    if (!rootView->GetParent()) {
-      PRBool widgetIsTransparent = PR_FALSE;
+  if (NS_GET_A(canvasColor.mBackgroundColor) < 255) {
+    // If the window is intended to be opaque, ensure that we always
+    // paint an opaque color for its root element, in case there's no
+    // background at all or a partly transparent image.
+    //
+    // The default background color from the prescontext is under user
+    // control, so it might not be opaque either.
+    nsIView* rView;
+    vm->GetRootView(rView);
+    if (!rView->GetParent() &&
+        (!rView->HasWidget() ||
+         rView->GetWidget()->GetTransparencyMode() == eTransparencyOpaque)) {
+      nscolor backColor =
+        NS_ComposeColors(NS_RGB(255,255,255),
+                         aPresContext->DefaultBackgroundColor());
 
-      if (rootView->HasWidget())
-        // We don't want to draw a bg for glass windows either
-        widgetIsTransparent = eTransparencyOpaque != rootView->GetWidget()->GetTransparencyMode();
-      
-      if (!widgetIsTransparent) {
-        // Ensure that we always paint a color for the root (in case there's
-        // no background at all or a partly transparent image).
-        canvasColor.mBackgroundColor = aPresContext->DefaultBackgroundColor();
-      }
+      canvasColor.mBackgroundColor =
+        NS_ComposeColors(backColor, canvasColor.mBackgroundColor);
     }
   }