[skrellm]

I've created a similar (minimal, lightweight, ANSI C) UI library, which is even easier to use.

https://gitlab.com/bztsrc/smgui

I don't like immediate-mode, because you must handle everything in the same thread, so mine is a state-mode GUI.

For immediate-mode and ANSI C, there's also Nuklear:

https://github.com/Immediate-Mode-UI/Nuklear

This latter has all the bells and whistles.