Debian GNU/Linux
Compose key (2021,2024)
"A compose key (sometimes called multi key) is a key on a computer keyboard that indicates that the following (usually 2 or more) keystrokes trigger the insertion of an alternate character, typically a precomposed character or a symbol.
For instance, typing Compose followed by ~ and then n will insert ñ."
On KDE, in 2024, System Settings > Input Devices > Keyboard > Advanced > Position of Compose key, I check Caps Lock. The Caps Lock key is the compose key now (and the Caps Lock light indicator is not turn on when the key is pressed).
On LXQT, in 2021, I used this command to map the compose key on right control key:
xmodmap -e "keysym Control_R = Multi_key".
It is necessery to add this line in the /etc/environment file:
GTK_IM_MODULE="xim"