bug#74617: 30.0.92; ffap-menu always displays the *Completions* buffer

[Daniel Mendler via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Daniel Mendler <mail@daniel-mendler.de>
>> Cc: 74617@debbugs.gnu.org,  monnier@iro.umontreal.ca
>> Date: Sat, 30 Nov 2024 17:25:07 +0100
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> >> I see it differently. The problem is in the tmm and ffap commands which
>> >> lead to a mixture of completion UIs. Even if another completion UI is in
>> >> effect, the default completion UI is called by tmm and ffap, bypassing
>> >> the `completing-read' abstraction.
>> >
>> > I don't understand: ffap-menu calls completing-read, so what
>> > abstraction does it bypass, and how?
>> 
>> It also calls `minibuffer-completion-help' which belongs to the default
>> completion UI but not strictly to the abstract `completing-read' API.
>
> So the problem is this single line?
>
>       (let ((minibuffer-setup-hook 'minibuffer-completion-help))

Yes.

But maybe it makes sense to tackle the "immediate candidate display
problem" more generally. It would be great to get Stefan's input on
this.

For example in my osm.el package I have the following code to implement
immediate display of candidates. The lambda implements the
auto-detection, which I don't find particularly elegant.

(minibuffer-with-setup-hook
    (lambda ()
      (when (and (eq completing-read-function #'completing-read-default)
                 (not (bound-and-true-p vertico-mode))
                 (not (bound-and-true-p icomplete-mode)))
        (let ((message-log-max nil)
              (inhibit-message t))
          ;; Show matches immediately for default completion.
          (minibuffer-completion-help))))
     (completing-read ...))

Initially I had forgotten the call to `minibuffer-completion-help' since
most of them time I use a completion UI which displays the candidates
automatically anyway. Then Richard reported the bug, that `osm-search'
doesn't work conveniently in Emacs by default. Furthermore in my jinx.el
package I have almost exactly the same code in `jinx--correct-setup',
which is used like this:

(minibuffer-with-setup-hook
    #'jinx--correct-setup
  (completing-read ...))

Daniel