title: GWT RPC possible in window closing handler?
body: <p>I'm listening for a window close event:</p>
  
  <pre><code>closeHandlerReg = Window.addCloseHandler(new CloseHandler&lt;Window&gt;() {
    @Override
    public void onClose(CloseEvent&lt;Window&gt; event) {
      // ...
    }
  });
  </code></pre>
  
  <p>The documentation says that no UI can be shown in that callback. What about GWT RPC calls? I'm trying to make one, but it isn't showing up on the server (either in breakpoints or log statements).</p>