void setWindowRegion(in nsISupports window, in nsISupports region)
Set a region to a window This method assigns a region to a window at the OS level. You should first construct an nsIRegion object, and pass it alongside the window to which you wish to assign the region.
Window region service interface This is an interface to the window region service, used to assign a region to a window at the OS level. This allows cutting out portions of a window without relying on mozilla's internal transparency management, which may interfere with the window content.