First of all, I am only a user of FLTK and not an active developer.
So this is just my personal opinion.
I am somewhat skeptical about the additional reactive layer.
The approach is interesting, but also requires a rethink in development.
The additional layer increases overhead and complexity.
FLTK is small and fast and user-based UI customization can be implemented quickly, but these are the things I like most about FLTK.
The development time can be shortened by using the active layer concept, but is it worth it?