In short, you can`t, without a very cooperative window manager (mwm will stack dialogs on the main window, but you can`t enforce stacking order among applications). Imagine the case in which two applications are trying this trick.

If you are using this top-most window to show errors or something else of great importance, then you can echo the information onto the main page in the MessageArea; you should not design your application to depend on a stacking order.

There is more information in the comp.windows.x FAQ, last seen on http//www.faqs.org and http//www.x.org.