frfrrf

Compare dependencies:

GTK3 https://gitlab.archlinux.org/archlinux/packaging/packages/gt... GTK4 https://gitlab.archlinux.org/archlinux/packaging/packages/gt... and FLTK https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=fltk-...

If you build FLTK with Wayland support, then add deps: wayland, libxkbcommon, pango.

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!