Welcome to fvwm. Fvwm is a multiple large virtual desktop window manager, originally (a looooong time ago!) derived from twm.
Fvwm is intended to have a small memory footprint but a rich feature set, be extremely customizable and extendible, and have a high degree of Motif mwm compatibility.
ALL VERSIONS OF FVWM PRIOR TO 2.6.X ARE NOT SUPPORTED UPSTREAM.
Users of fvwm prior to 2.6.X can make use of the [./bin/fvwm-convert-2.6.in](fvwm-convert-2.6 utility) to try and have their configuration file converted to a newer syntax. Please see its manpage for more details on how to use it.
Those interested in contributing to FVWM should have a read of the developer documentation.
A TODO file exists, and sometimes even things from it are worked on.
If you have any questions, concerns, bug reports, enhancement requests, etc., please feel free to send an email to the fvwm mailing list, or open a Github issue.
Have fun!
-- The fvwm-workers