bug#52389: 29.0.50; Imenu does not pop up the Completions eagerly

[Juri Linkov <juri@linkov.net>]

>> This is a good opportunity for generalization.
>> Like there is icomplete-show-matches-on-no-input,
>> would it be possible to create a new user option
>> e.g. minibuffer-auto-show-completions, and allow
>> to customize what commands should use it.
>> Then it could be like completion-auto-select
>> recently proposed by Philip.
>
> That is possible with my mct package on GNU ELPA.  It has the option of
> a passlist for commands that should eagerly pop up the Completions'
> buffer.
>
> [ Philip mentioned mct in those patches. ]
>
> Note, however, that the case here with Imenu is different because even
> if you pop the completions eagerly upon M-x imenu, you still need to
> handle the case of nested Imenu entries.  That is what the user option
> imenu-eager-completion-buffer helps deal with.  Actually, what prompted
> me to report this bug was due to an issue in the mct repo, where I
> discovered the confusing doc string of that Imenu user option:
> <https://gitlab.com/protesilaos/mct/-/issues/12>.

Thanks for the reference, I see the problem now: after adding ‘imenu’ to
mct-completion-passlist, ‘M-x imenu RET’ auto-displays completions
correctly.  But the problem is that selecting e.g. a nested completion
doesn't auto-display the nested completion list, because in the first
case ‘this-command’ is ‘imenu’, but in the second case it's a command
that selects the nested completion.

Fortunately, it's possible to solve this problem using
the new variable introduced in 28.1: ‘current-minibuffer-command’
that is like ‘this-command’, but preserves its value
during the nested minibuffer activities.  To make it
backward-compatibile with Emacs 27.1, 'bound-and-true-p'
could help to check if the variable is defined, e.g.

#+begin_src emacs-lisp
   ((memq (or (bound-and-true-p current-minibuffer-command)
              this-command)
          mct-completion-passlist)
#+end_src

Then 'imenu' in 'mct-completion-passlist' will be available
for the nested minibuffer commands.

PS: Overall, your package mct makes progress towards better
minibuffer/completions interactions.  However, its behaviour
can't be enabled by default in Emacs, because e.g. such keys
as up/down arrows traditionally are used for history navigation.