I'm not sure this is helpful (yet?), but the Red programming language seems to be working on cross-platform UI support. Original announcement is here, and macO

I'm not sure this is helpful (yet?), but the Red programming language seems to be working on cross-platform UI support. Original announcement is here, and macOS support was recently added. Language development seems pretty slow, though. http://www.red-lang.org/2016/03/060-red-gui-system.html?m=1 http://www.red-lang.org/2016/03/060-red-gui-system.html?m=1