If I run the following code, then I get a centered popup box. However, If the Browser window is scrollable (i.e. the visible area is smaller than the total area displayed), the box appears at some strange positions. This also happens for tooltips and Windows.

Code:
MessageBox.alert("bla", "bla", new Listener<WindowEvent>() {
                public void handleEvent(WindowEvent be) {
                }
});