Regarding FLTK: was originálly done by Bill Spitzak (who obviously has used IRIX/4DWM), now famous for Nuke https://www.atogt.com/askoscar/display-person.php?id=74125&v...
Meanwhile, XForms has been open-sourced, but FLTK being in a different language and having evolved a bit since its creation didn't suffer from the problems Lesstif had and is standing on its own rather well. And despite being C++, it pops up rather often when you're looking for GUIs with decent language bindings (e.g. for Lua or Rust). Probably because it's less a moving target than Qt or, heck, Gtk.
Good times.