Eli Zaretskii:
>> In that case, most uses of icomplete-vertical and any similar features 
>> "have bugs".  There's no chance in the world you can display all the 
>> files in a non-trivial directory vertically in a mini-window without 
>> losing some of the file names and/or some of the prompt.

Luckily, there's a chance that you can display the entire prompt! (*)

martin rudalics:
> Right.  I exclusively use systems where I can always see the entire 
> prompt which might ask me something like "Delete 27 files?" and 
> additionally allows me to see the names of these files via some extra 
> scrolling mechanism.  Provided there's not enough space to see the 
> entire prompt and the names of all these files in a popped-up window. In 
> either case, the entire prompt must be visible.

I fully agree with you that "the entire prompt must always be visible". 
Icomplete does this, with the attached code.  The entire prompt is always 
visible (*), and you can scroll the completion candidates with C-. and 
C-,.

(*) Except of course in pathological cases, for example with (setq 
max-mini-window-height 1) and (set-frame-width nil 10).  But in these 
cases nothing can be done.