Bug 624452. Report the menu background as opaque if theming of it is not supported so it gets subpixel AA. r=roc a=blocking

widget/src/windows/nsNativeThemeWin.cpp

@@ -2243,8 +2243,13 @@ nsNativeThemeWin::GetWidgetTransparency(nsIFrame* aFrame, PRUint8 aWidgetType)
 
   HANDLE theme = GetTheme(aWidgetType);
   // For the classic theme we don't really have a way of knowing
-  if (!theme)
+  if (!theme) {
+    // menu backgrounds which can't be themed are opaque
+    if (aWidgetType == NS_THEME_MENUPOPUP) {
+      return eOpaque;
+    }
     return eUnknownTransparency;
+  }
 
   PRInt32 part, state;
   nsresult rv = GetThemePartAndState(aFrame, aWidgetType, part, state);