@robert.ancell Thanks for reviewing - it looks like that commit relates to the AltGr ("3rd Level") settings rather than Compose key settings. Indeed, XKB doesn't define a way to use Left Ctrl as AltGr, but it does define the compose:lctrl
option:
$ grep -F compose: /usr/share/X11/xkb/rules/*.lst
# ...
/usr/share/X11/xkb/rules/base.lst: compose:lctrl Left Ctrl
Adds an option to set Left Ctrl as the Compose key via the Keyboard panel. Resolves #1860.
Note that I targeted this MR to gnome-42, hoping that this could go out in a minor release, but I can rebase onto main if this isn't appropriate.
(Build appears to be failing because I made my fork Internal rather than Public. Will resolve that shortly.)
Kevin Bullock (a3a65ce3) at 19 May 02:41
keyboard: Add Left Ctrl to Compose Key options
... and 15 more commits
In Ubuntu 20.04 (before !785), I was able to set Left Ctrl as my Compose key using gnome-tweaks. I just upgraded to 22.04, where the Compose key settings have been removed from gnome-tweaks and moved into control-center, but Left Ctrl is not in the list of options for Compose.
I was able to restore Left Ctrl as my Compose key using:
$ gsettings set org.gnome.desktop.input-sources xkb-options ...
adding 'compose:lctrl' to that list, but it would be nice to expose it in the GUI.
For context, my modifier settings are:
which gives me both a Ctrl key on both sides of the keyboard and an easily-accessible Compose key.
Originally reported to Ubuntu as https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/1973226. I've set this as a bug because it caused a regression for me — my modifier settings were lost after upgrade. I could understand reclassifying it as a feature request if that's desired.