New DialogBox underneath the prevous DialogBox!!

According to popuppanel (and so DialogBox) api,

A panel that can "pop up" over other widgets. It overlays the browser's client area (and any previously-created popups).

but I found that the new DialogBox is underneath the origin DialogBox, why?

for the original dialogbox, I set

original.setGlassEnabled(true);
original.setAnimationEnabled(true);
original.center();

the original dialogbox has a button to call the new DialogBox(callee), I set

callee.setModal(false);
callee.setGlassEnabled(true);
callee.setAnimationEnbaled(true);
callee.center();

the result is the new DialogBox show underneath the old DialogBox!!

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