For some other purposes I have been looking into the old XForms library for Linux/Unix, and I realized that it can easily be used by newLisp. :-)
No external software is needed, you can use XForms directly, out-of-the-box, straight from the library, including a fully functional callback mechanism.
Below the XForms context, a demoprogram and screenshot.
(For me and newLISP fellow 'Sam' it is the real powerfull combination, easy but powerfull GUI powered by the wonderfull newLISP)
(Not free but worth every $/€)
I notice a distinct absence of a certain OS of the fruit family :-)
I've imagined a few times what a newTk for newLISP could be about. Something in the same spirit as newLISP (simple, small, agile, cooperative, and adapting). Based on OpenGL, perhaps? (Have you seen Blender's interface? It convinced me powerful GUIs can be implemented in OpenGL.) And the best part is, we could get the Tk people mad at us, too! Just like the oldLISPers ;-)