sourceforge does ship 1.3.7 [1] from 2016 and there is fluxbox.git [2]

QT5 apps are acting up in fluxbox, when you try to leave main windows in
maximized state. A fix for that was discussed and proposed in the
sourceforge bug tracker [3].

Proposed patch is small and applies fine against 1.3.7 or 1.3.7-git

Please consider it as an upgrade for Bullseye - it provides hassle free
use of kwrite, konsole, even vlc in a fluxbox environment, given that
qt5ct is installed as well.

Thank you!

[1] https://sourceforge.net/projects/fluxbox/files/latest/download
[2] http://git.fluxbox.org/fluxbox.git/
[3] https://sourceforge.net/p/fluxbox/bugs/1176/#cc9f