OT: M-S-$ Not Working

[Bo Grimes <vcg3rd@fastmail.com>]

I beg your indulgence.  I am confident this isn't an Emacs problem, let
alone an org problem, but my eyes hurt from searching for an answer,
and this list, the only one I subscribe to, is populated with gurus.  I
promise never to use it this way again.

OS: PopOS 20.10, DE: GNOME 3.38.2 WM: Mutter 
GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.14)

M-S-$ does not spellcheck word.  It will work from the menu bar, and it
will work if I drop into a tty and run Emacs.  Emacs gives no
response in the minibuffer in the GUI when I press M-S-$. M-x
describe-key M-S-$ does nothing. C-h b C-s 'spell' reveals that indeed
M-S-$ is bound to spellcheck word. And other M-S- keys work like M-S->
just fine.

There has to be some keybinding outside Emacs taking precedence. I have
gone through dconf-editor until my eyes bleed. Done gsettings
list-recursively  org.gnome.desktop.wm.keybindings | sort | more and
gone line by line.  I have done dconf dump / > dconf.dump and read
through them all, in addition to checking PopOS' keybindings in
Settings. And trying a different keyboard.

Nothing in Tweeks, dconf, or Settings uses M-S-$, but I disabled
anything that uses Shift anyway (nothing uses $). No joy. I don't want
to rebind it for this machine only, nor do I want to go through the
hassle of installing a different DM/WM.

StackExchange, et.al are full of problems with the the M key, but not
one specific keychord only.

Any ideas? I will accept rebukes.  I'm desperate.  TIA!

Bo Grimes