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

@@ -1159,6 +1159,13 @@
     }
 
     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);
+    }
 }