bug#59314: 29.0.50; EUDC and message-mode header completion

[Alexander Adolf <alexander.adolf@condition-alpha.com>]

I have done some further debugging, and it seems that the behaviour is
independent of EUDC.

With this:

(setq message-expand-name-standard-ui t)
(setq eudc-server nil eudc-server-hotlist nil)
(setq message-expand-name-databases '(bbdb))

and `eudc-capf-complete` removed from `completion-at-point-functions` in
`message-mode`, the result is the same (completion is broke).

It seems that the completion styles are attempting to find the longest
common prefix of all candidates, and somehow take words into account.
Quoting from the Elisp manual [1]:

---------------------------- Begin Quote -----------------------------
The procedure of determining what constitutes a match is quite
intricate. Emacs attempts to offer plausible completions under most
circumstances.
----------------------------- End Quote ------------------------------

Have I just been handed a word of warning? Looks like.

Anyways, I will now try to see whether I can identify a commit that the
issue. I'll start with the one mentioned by Eric.

[puts on helmet, switches on headlamp, and grabs a pickax]

Wish me luck,

  --alexander