I am now using MATE as DE for my EOS installation, but I have seen the same option in KDE Plasma, XFCE, and GNOME (if I recall correctly).
In Keyboard Preferences (Layouts) there is a button to set further options (Options…) which shows a dialog box (Keyboard Layout Options) with the following options (among others).
On Position of Compose key, I choose the Pause key; then, in Use keyboard LED to indicate modifiers, I chose Compose, which is the only option available.
I thought that would switch on a LED when the keyboard was on Compose mode, but that does not seem to happen.
What is the purpose of the Use keyboard LED to indicate modifiers options? (I take I misunderstood it.)
On my keyboard (a Logitech G105), I have three LEDs: Caps Lock, Scroll Lock, and Numeric Lock. None of them switches on when I press the Pause key.
It should work. These settings are for Xorg, not Wayland, so Gnome would not use them AFAIK. At least this is what my Gnome Wayland session does, while my bspwm session switches on the Scroll LED (in my case) when I switch keyboard layout (one option lower than yours in the posted image).
The output of localectl command should reflect the GUI settings (as posted). Also the Xorg conf file for keyboard at /etc/X11/xorg.conf.d/.
For the Compose (only) option, I suppose that an equivalent LED should exist (like the Scroll LED in my case).
If I understood what you are saying, with that option, the Compose LED would be switched on when I press the Compose key (whichever key I set as Composer key). Is that correct?
Yes, that’s how I understand the function. I suppose there must be some keyboards with a Compose LED, or some xmodmap rule that may assign this role to an existing LED.
Basing on Wikipedia, modern keyboards do not have neither the Compose key nor the Compose LED, but it could be xmodmap could have a rule to make a different LED the one to use for that key.
