Unfortunately, FLTK falls short in at least one important way: it doesn't implement accessibility, e.g. for blind users with screen readers. Godot and immediate-mode GUIs have the same problem.
If I'm not mistaken, the most lightweight cross-platform UI toolkit that draws its own widgets (unlike wx or SWT) and implements accessibility is sciter [1]. It's a good thing the antivirus vendors (who seem to be sciter's biggest customers) want to sell to governments.
That's a closed source for pay gui built on html and css.
It probably would not be too difficult to hook into FLTK's label drawing, but this is a general comment about typical small to medium sized guis, originally as a reply to someone needing an internal tool built around an openGL window. Maybe it would be better to make a top level comment and mention that the game engine isn't accessible to blind users.
If I'm not mistaken, the most lightweight cross-platform UI toolkit that draws its own widgets (unlike wx or SWT) and implements accessibility is sciter [1]. It's a good thing the antivirus vendors (who seem to be sciter's biggest customers) want to sell to governments.
[1]: https://sciter.com/