MacOS

Compose Key

1. Install Karabiner-Elements, e.g. with brew install.
2. Fetch the large but comprehensive DefaultKeyBinding.dict file from GitHub, here. This has all the X11 compose key mappings in it, plus a bunch of other nifty combinations.
3. . Copy it into the ~/Library/KeyBindings directory. You may have to create the directory first.
4. In Karabiner-Elements under Settings → Simple Modifications, add the following remapping rules for any USB keyboards:
   • left_command → left_alt
   • left_alt → left_command
   • right_alt → right_command
5. In Settings → Complex Modifications, Add the rule to rewrite the Right Command key (now mapped as Right Alt on USB keyboards) to Shift + Ctrl + F13. This rule should look like this:

{
  "description": "Send Shift + Ctrl + F13 on Right Command to emulate Compose Key",
  "manipulators": [
    {
      "from": { "key_code": "right_gui" },
      "to": [
        {
          "key_code": "f13",
          "modifiers": ["left_control", "left_shift"],
          "repeat": false
        }
      ],
      "type": "basic"
    }
  ]
}

1. Possibly restart apps if they don’t immediately pick up changes.

References

1. Goller, A (16 August 2022). “Compose Key on MacOS” (blog post), alexandergoller.com
2. “Send Shift + Ctrl + F13 on Right Command to emulate Compose Key”, Gene Sy:Karabiner Complex Rules Generator, via GitHub.
3. Hocevar, Sam (17 June 2012). “Setting up a real Compose key on Mac OS X” (blog post), The Lol Engine (dead link; archived 20 October 2021 by The Internet Archive).