Sure that works. I am uncertain however if it is "the right thing" (r) to do. Note, that in conjunction with eg. marginalia, the information to an entry can become quite long, hopefully because it provides helpful information. This added information would no longer be visible and I don't know how to scroll to the left / right to make it visible.

Additionally, if you press two times <TAB> <TAB> (requiring  '(completion-auto-select 'second-tab) to be set) you get a different (arguably better) scrolling behaviour, breaking long lines while retaining the whole information. Similar functionality, different code?

Sidenote: fido-vertical mode makes little sense in conjunction with (completion-auto-select 'second-tab) but I get what I ask for.

Am Do., 2. März 2023 um 12:57 Uhr schrieb João Távora <joaotavora@gmail.com>:
On Thu, Mar 2, 2023 at 11:52 AM Eli Zaretskii <eliz@gnu.org> wrote:
>
> > From: Johann Höchtl <johann.hoechtl@gmail.com>
> > Date: Thu, 23 Feb 2023 08:12:09 +0100
> > Cc: 61702@debbugs.gnu.org
> >
> > emacs -Q
> > M-x fido-vertical-mode
> > M-x <consta> <-- any search term to narrow down the potential completions, in this case 12 items remain
> > matching
> > narrow the whole emacs window so the search results have to "break" because of long lines
> > <down> <down> ...
> > The highlighted active line remains visible until the last items, than the active line becomes invisible
>
> Thanks.
>
> It looks like the code in icomplete--render-vertical implicitly
> assumes that every candidate takes just one screen line, which is
> false in your scenario.  A workaround is to set truncate-lines non-nil
> in the minibuffer.
>
> João, can you take a look, please?

I'll take a better look later, but I can say that that truncate-lines
idea sounds very sensible.  Johann can you try this patch?

diff --git a/lisp/icomplete.el b/lisp/icomplete.el
index 014f38b2024..4e85e20fddb 100644
--- a/lisp/icomplete.el
+++ b/lisp/icomplete.el
@@ -644,6 +644,7 @@ icomplete--vertical-minibuffer-setup
   (setq-local icomplete-hide-common-prefix nil
               ;; Ask `icomplete-completions' to return enough
completions candidates.
               icomplete-prospects-height 25
+              truncate-lines t
               redisplay-adhoc-scroll-in-resize-mini-windows nil))

 ;;;###autoload