src/macosx/classes/sun/lwawt/macosx/LWCToolkit.java

*** 718,726 ****
--- 718,735 ----
      public boolean isTranslucencyCapable(GraphicsConfiguration gc) {
          return true;
      }
  
      @Override
+     public Color getLayerBackground(Window w) {
+         if (!w.isOpaque()) {
+             return new Color(0f, 0f, 0f, 0f);
+         } else {
+             return w.getBackground();
+         }
+     }
+ 
+     @Override
      public boolean enableInputMethodsForTextComponent() {
          return true;
      }
  }