> Is Mod5Mask always AltGr (on my system it is)?
> The existing code calculates m_altgrMask.
on my system it's also Mod5Mask, but I have found code such as
  ftp://ftp.fvwm.org/pub/fvwm/devel/sources/libs/modifiers.c
where AltrGr is hardcoded to Mod3Mask...