strange behaviour: Emacs ignoring one key on my keyboard

[Eric S Fraga]

Hello,

I am very perplexed 🤔 and I'm hoping somebody might be able to help.

For (many) years now, I have defined the left "Windows" key on my
keyboards to act as F20 using

$ xmodmap -e "keycode 133 = F20"

and have then bound whatever function I wanted to F20 in my Emacs
startup.  This has always worked.

Recently (in some definition of recent, which could be several months),
Emacs (from git, up to date as of an hour or so ago) has started to
ignore that key completely.  C-h c doesn't respond when I hit the key.
Other applications see the key when pressed, including xterm and in
particular xev which displays it correctly as F20:

,----
| KeyPress event, serial 36, synthetic NO, window 0x2800001,
|     root 0x5b3, subw 0x0, time 630914892, (410,577), root:(2981,578),
|     state 0x0, keycode 133 (keysym 0xffd1, F20), same_screen YES,
|     XLookupString gives 0 bytes: 
|     XmbLookupString gives 0 bytes: 
|     XFilterEvent returns: False
`----

I do similar for the "Menu" key on the right of the keyboard and this
works just fine, defining it to be F21.  The xev output is very similar:

,----
| KeyPress event, serial 36, synthetic NO, window 0x2800001,
|     root 0x5b3, subw 0x0, time 630974382, (1812,1286), root:(4383,1287),
|     state 0x0, keycode 135 (keysym 0xffd2, F21), same_screen YES,
|     XLookupString gives 0 bytes: 
|     XmbLookupString gives 0 bytes: 
|     XFilterEvent returns: False
`----

I cannot find anything in my .emacs (etc.) that would cause this
difference in behaviour between the 2 keys; the window manager (stumpwm)
cannot be interfering if other applications see the key (or so I
assume).

Any suggestions or pointers for further investigation welcome!

Thank you,
eric

-- 
Eric S Fraga with org 9.5.2 in Emacs 29.0.50 on Debian 11.3