Summary: I attempted to define some keybindings using the "5" modifier key in my fvwmrc file, after using xmodmap to map the Windows key to Mode_switch (which according to xmodmap is mod5), and they don't seem to work. Here's one of the non-functional fvwm keybindings:
Key M A 5 Move
Starting from the beginning to explain what I've been doing and why - originally I had the fvwm keybindings using mod4:
Key M A 4 Move
This worked fine (using the Windows key, which by default appears to be Super_L, which according to xmodmap is mod4).
What I'm trying to do is re-map the Windows key to be Mode_switch instead of Super_L, and change my fvwm keybindings to match so that they still work the same way as before. The reason I'm doing this is that I want additional keybindings such as Windows+J = Down, and it looks like I can't do that naively with xmodmap because xmodmap doesn't let me redefine keys with the Super_L modifier (or the Ctrl or Alt modifiers), only Shift and Mode_switch and ISO_Level3_Shift - so I figured I could change the Windows key to be Mode_switch and that way I'd be able to use it both in xmodmap remapping and in fvwm keybindings. If anyone has better ways of getting those or similar keybindings that doesn't involve remapping Windows to Mode_switch, let me know.
Here's the xmodmap command I used to set the Windows key as Mode_switch:
xmodmap -e "keycode 133 = Mode_switch NoSymbol Mode_switch"
According to xev, this worked (when I press the Windows key while running xev, it shows
keycode 133 (keysym 0xff7e, Mode_switch))
So, if the Windows key is Mode_switch, and Mode_switch is a mod5 key (which is what the output of
xmodmap lists it as), why doesn't my new fvwm keybinding using the "5" modifier trigger when I press Windows+M?
I don't know if this is a problem with xmodmap, with fvwm, or if there's some completely different reason why the thing I'm trying to do can't work.
This is all on Xubuntu 20.04 with fvwm as the window manager, standard 105-key US keyboard layout with almost no other xmodmap changes (only remapped the PrintScreen key and F12), no xkb changes, not sure what other information is relevant.