Found in GWT Release: GWT1.4

Detailed description:

Doing the following to position a popup panel in the correct position 
relative to the browser window in GWT1.4.
int x = 0, SOMEPOSX = SOMEPOSY;
 public void onLoad() {
            super.onLoad();
                    if (x + (2*getOffsetWidth()) > Window.getClientWidth
()) {
                        x = Window.getClientWidth() - (2 *getOffsetWidth
());
                    }
                    if (y + (2*getOffsetHeight()) > Window.getClientHeight
()) {
                        y = Window.getClientHeight() - (2*getOffsetHeight
());
                    }
                    setPopupPosition(x, y);
               
        }

This is because getOffsetX() methods return 0 when called from inside the 
onload() method. In GWT1.3 and prior releases, these methods returned the 
actual size of the panel but has become broken in GWT1.4 

Workaround if you have one:
Use DeferredCommand.addCommand() thus:

public void onLoad() {
            super.onLoad();
            DeferredCommand.addCommand(new Command() {
                public void execute() {
                    if (x + (2*getOffsetWidth()) > Window.getClientWidth
()) {
                        x = Window.getClientWidth() - (2 *getOffsetWidth
());
                    }
                    if (y + (2*getOffsetHeight()) > Window.getClientHeight
()) {
                        y = Window.getClientHeight() - (2*getOffsetHeight
());
                    }
                    setPopupPosition(x, y);
                }
            });
        }

Links to the relevant GWT Developer Forum posts: