It’s connected with the Lucid toolkit, but only in an indirect causal sort of way:— I’m using the standard GTK3 toolkit on this particular build of Emacs on Guix. So, skipping over large swathes of story, I’ve got a seemingly mostly-functioning build of Emacs 30.0.93 running on my Guix laptop (which finally has a functioning StumpWM environment again), and I go to type æ or þ or á or λ or something using Right_Alt as a compose key (defined by xmodmap ) and Emacs, without blinking an eye, flashes at me, saying "<Multi_key> is undefined" . But I don’t think that would quite work anyway, since sometimes I want to say insert a special character as part of a search and not insert it into a text buffer.