We are working on GUI testing of Forte for Java by Jemmy (GUI testing tool, see http://techpubs.Eng.Sun.COM:80/net/ultrapuma/export/ultrapuma4/jemmypages/ if you are interested). We found out that there can exist JPopupMenu which has isShowing()==true, isVisible()==false, popup==null.
I think this is the state in which no component should stay because javadoc of Component.isShowing() says:

    /**
     * Determines whether this component is showing on screen. This means
     * that the component must be visible, and it must be in a container
     * that is visible and showing.
     */

I guess isShowing() method should be implemented in JPopupMenu class such way as isVisible() method, i.e.

    public boolean isShowing() {
        if(popup != null)
            return true;
        else
            return false;
    }
    
Then our tests will find the correct popup-menu.