Plain and simple C99+ port?
Language with OOP is good for GUI!
C89 bindings https://github.com/MoAlyousef/cfltk
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)
Basically, to be fair, in qt5 and fltk cases you should add an amplification factor of at least "10" (probably not big enough since c++ syntax and tens of times more complex than plain C99+).
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.