I haven't try it, but code (for JPopupMenu) says that pack() calls validate.

That is why I believe revalidate() will do the job too.
[doc for JPopupMenu revalidate()]
This method will automatically be called on this component
     * when a property value changes such that size, location, or
     * internal layout of this component has been affected.
This automatic updating differs from the AWT because programs generally
no longer need to invoke <code>validate</code> to get the contents of
the  GUI to update.
[/doc]

j2sdk1.4.2_04 (JPopupMenu)
[code]
public void pack() {
        if(popup != null) {
            Dimension pref = getPreferredSize();

            if (pref == null || pref.width != getWidth() ||
                                pref.height != getHeight()) {
                popup = getPopup();
            } else {
                validate();
            }
        }
    }
[/code]