Compose key and i3

The compose key on Linux and other systems enables you to enter symbols like ™ that don't have a dedicated key on most keyboards. GNOME makes this easy to do via Gnome Tweaks. However, using a window manager like i3 means there's no dedicated GUI for changing the hotkey for compose.

This stackexchange answer sent me down the right path of adding the following config to ~/.Xmodmap to bind Caps lock to compose:

.keysym 66 = Mode_switch
clear Lock

You can then enable the key map by running:

xmodmap ~/.Xmodmap

However, that config didn't work because Mode_switch wasn't the right name for the compose key (at least under Ubuntu/PopOS 20 for me). The Arch Wiki states that the name for the compose key is actually Multi_key. With this new name I was able to get Caps lock to act as the compose key by updating the config in ~/.Xmodmap to:

keycode 66 = Multi_key
clear Lock

The final step is to set that mapping every time i3 runs by putting this inside your i3 config file like so:

exec --no-startup-id "xmodmap ~/.Xmodmap"