setDraggable(false) not working on Portlet-Window

When i open a new Window in a portlet Application running in Liferay calling setDraggable(false) does not prevent the Window from being dragable. I tested a simple Window in a standalone Vaadin App and it behaves as expected.

Can anybody tell me how to prevent a Window (modal Dialog) from being dragged around in a portlet app?

Reagrds Michael