John, could you check whether this still occurs with fldigi 3.21.4-1
(unstable)?
FWIW, I was unsuccessful in my attempt to replicate (something like) the
problem in the new 3.21.4 or the old 3.20.23... I tried externally
changing the main fldigi window geometry with the program
'xlayout' (built from google-located source) but could not make any
problem occur. The fldigi window responds to the the external geometry
changes; it limits the minimum window height and width to some apparent
internal minimum values but never hangs or crashes. Maybe xmonad
changes the window geometry some other way (?).