Style "lt-fontforge" !UseUSPosition, !UseTransientUSPosition
in my .fvwm config file. One thing of note is the string "lt-fontforge". I thought the first argument of the fvwm Style command had to be a string matching the title of a window, which would be a real pain in the case of fontforge, because it does not include any one reliable substring in its windows' names. Fortunately, Style is actually matching (according to the manpage) "a window's name, class, visible name, or resource string", with "class" and "resource string" being afaict conventionally application-consistent strings supplied to X11 somewheres in the guts of the window creation process. I was able to find out the one fontforge uses by running FvwmIdent. Dunno what "lt" stands for.
Again citing the manpage, what !UseUSPosition does is it
suppresses using the user specified position indicated by the program (USPosition hint). It is generally a bad thing to override the user's choice, but some applications misuse the USPosition hint to force their windows to a certain spot on the screen without the user's consent.
with the arch passive-agressive bold emphasis being entirely mine. I'm looking at you, fontforge.
But of course !UseUSPosition doesn't suppress this bad behavior for all windows, just all nontransient windows. You need to additionally say !UseTransientUSPosition for that. Not that the documentation tells you this or anything, but you can infer it from the documentation for UseTransientPPosition if you happen to look there.
Mysteriously, once I did "make install", lt-fontforge changed to just fontforge.