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

*** 1159,1164 ****
--- 1159,1171 ----
      }
  
      private boolean isGrabbing() {
          return this == grabbingWindow;
      }
+     
+     @Override
+     public void setBackground(final Color c) {
+         // draw pure transparent pixels into the CALayer as the background color
+         Color bgColor = ((SunToolkit)Toolkit.getDefaultToolkit()).getLayerBackground(getTarget());
+         super.setBackground(bgColor);
+     }
  }