src/share/classes/sun/swing/LightweightContent.java

@@ -177,6 +177,14 @@
     /**
      * {@code JLightweightFrame} calls this method to notify the client
      * application that the content minimum size has changed.
      */
     public void minimumSizeChanged(int width, int height);
+
+    /**
+     * Invokes a runnable on the client's toolkit thread
+     * @param r a runnable to invoke
+     */
+    default public void invokeOnClientToolkitThread(Runnable r) {
+        throw new UnsupportedOperationException("Unsupported client implementation");
+    }
 }