No, there's no real progress. The Fl_Flex widget is IMHO not
feature-complete and it uses (IIRC) std::vector which we can't use
in FLTK.
Making a solid row/column (or grid) widget will take some time. It
should be much better (usable) than Fl_Pack and needs a flexible API
from the beginning which can't be developed in a few hours/days. But
if we add a new widget it should have an API that is ready to go and
(almost) feature-complete. It's hard to do it right.
If you're interested in a quick solution you could use either
Fl_Pack or Fl_Flex, whichever satisfies your needs.
For a long-term solution please watch the two related GitHub issues.
Progress will show up there: