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frfrrf

As for complexity and gigantism:

https://openhub.net/p/qt5 (8,876,034 lines of code)

https://openhub.net/p/gtk (1,031,203 lines of code)

https://openhub.net/p/fltk (467,564 lines of code)

>Plain and simple C99+ port?

C89 bindings https://github.com/MoAlyousef/cfltk

>Then it is c++, then a definitive nono.

Language with OOP is good for GUI!

Look at FLTK's source code. It's a minimal set of C++. Like C with classes.
>Planned support for systems in the near future: Linux GTK3/4

FLTK is better then GTK on linux. Since version 1.4 FLTK supports HighDPI displays and Wayland https://www.fltk.org/articles.php?L1947 GTK3/4 and Qt5/6 are bloatware!