IUP is very nice and easy to use, and it uses real native controls, which is a big plus. Compared to wxWidgets, being written in C, it is easy to create bindings. I maintain Go bindings for IUP core here https://github.com/gen2brain/iup-go.

I wish development is faster though, perhaps moving the project to GitHub could help with that. I would like to see modern tray support on Linux instead of legacy one, someone to finish Cocoa support, native table/list view control (it is on the TODO list), support for gtk+ 4, etc.