I'm late to the party here, but I'd just like to add that
Gregory's solution based on advice can easily be be integrated into
icomplete-mode directly.  I do like that it's such a simple solution,
doesn't include new defcustoms, modes and such.  I didn't have
a in-depth look, but it looks like your solution Ergus, is quite a bit
more complex.  Why?  What extras does it bring to the table?
And are we sure we need those extras in the first version of this?

Or am I missing something and is Gregory's solution fundamentally
unusable?

My 2c,
João

On Sat, Sep 12, 2020 at 2:33 PM Ergus <spacibba@aol.com> wrote:

> On Sat, Sep 12, 2020 at 01:10:57PM +0000, Gregory Heytings wrote:
> >
> >>
> >>>If there was a built-in vertical mode it would be better / more
> >>>intuitive.
> >>
> >>Could you try the branch feature/icomplete-vertical? I need some
> >>testers before adding it to master.
> >>
> >
> >Alas no, I have been using the following to have icomplete-vertical
> >for quite some time, it works perfectly well, so I don't see why a
> >more complex implementation would be necessary.
> >
> >(setq icomplete-prospects-height 6)
> >(setq icomplete-separator "\n")
> >(defun icomplete-vertical-minibuffer-setup ()
> >  (setq truncate-lines t)
> >  (setq-local completion-ignore-case t)
> >  (setq-local read-file-name-completion-ignore-case t)
> >  (setq-local read-buffer-completion-ignore-case t)
> >  (setq icomplete-hide-common-prefix nil))
> >(add-hook 'icomplete-minibuffer-setup-hook
> #'icomplete-vertical-minibuffer-setup)
> >(defun icomplete-vertical-reformat-completions (completions)
> >  (save-match-data
> >    (let ((cnp (substring-no-properties completions)))
> >      (if (string-match "^\\((.*)\\|\\[.+\\]\\)?{\\(\\(?:.\\|\n\\)+\\)}"
> cnp)
> >          (format "%s \n%s"
> >                  (or (match-string 1 cnp) "")
> >                  (replace-regexp-in-string "^" (make-string
> (current-column) ? ) (match-string 2 cnp)))
> >        cnp))))
> >(defun icomplete-vertical-adjust-minibuffer-height (completions)
> >  (let* ((comp (icomplete-vertical-reformat-completions completions))
> >         (complen (length (split-string comp "\n"))))
> >    (if (> complen 1) (enlarge-window (- icomplete-prospects-height (1-
> (window-height)))))
> >    comp))
> >(advice-add 'icomplete-completions :filter-return
> #'icomplete-vertical-adjust-minibuffer-height)
>
> 1) Internal functionalities try not to use advises.
> 2) The branch is not actually more complex, it just generates the
> formatted vertical output form the beginning.
> 3) It does more or less the same you are doing but with a simpler
> config:
>
> (icomplete-mode t)
> (icomplete-format 'vertical)
>
> 4) We add arrow bindings to move
> 5) Add completion matching faces is also coming.
>
>

-- 
João Távora