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

*** 177,182 ****
--- 177,190 ----
      /**
       * {@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");
+     }
  }