Messagebox onClose with event thread disabled

Hi,
when the event thread is disabled, then it is not possible to catch the event, when the user closes the dialogue by the click on the upper right cross. Try this example:

<button label="Warning">
<attribute name="onClick">{
Messagebox.show("Warning is pressed", "Warning", Messagebox.OK, Messagebox.EXCLAMATION, new EventListener() {
public void onEvent(Event event) {
alert("OK");
}});
}</attribute>

I would expect the onEvent() method to be called with ((Integer) event.getData()).intValue() equals to zero.