For some reason subscribing to list does not work for me, so I only saw your reply in archive ...
mgSTEP in this case surely was run on top of X11, but it can be switched compile-time to framebuffer.
speaking about modern Qt - it become ... a bit massive in last 25+ years ... so I do not thinking anything lighter will be "competing" with it
Perhaps i should put it on github, so internals will easier to discuss
Also, I was surprised someone did GUI/window (one window) demo in Forth!
"Figure 415 shows a rudimentary implementation in Open Firmware of a window that can be dragged.[15]"