This is fantastic. FLTK is great software and a very nice library to know. Now having wayland support and high dpi support, it will what people need in a more modern GUI library.
With previous versions you could get the pdf for all your documentation and start with a 100KB completely static binary that would show a window and buttons. The source is pretty clear so making your own components or modifying existing ones is straight forward and it really is lightweight and FAST.
It also has network stuff and jpg/png and audio libraries so checks off a few more potential dependencies. It has threading too, but that seems like less of a modern issue.
If someone was delivering software I would love it to use FLTK and be a single fast static binary. That's the kind of stuff that isn't a constant pain and can just work so you can move on to other problems.