Hi,

I'm a bit late to the icomplete-vertical party, but please let me show
you my approach at supporting "\n" in 'icomplete-separator'.

The main idea is to treat the 'icomplete-prospects-height' variable
value as the amount of completions we want to see if there is "\n" in the
'icomplete-separator'.

Please see attached patch.

Also see attached screenshot of minibuffer-only frame after:

emacs -Q --eval "(setq default-frame-alist '((minibuffer . nil)))"
M-x icomplete-mode
(setq icomplete-prospects-height 5)
(setq icomplete-hide-common-prefix nil)
(setq icomplete-separator "\n ")
(setq resize-mini-frames t)
M-x icomp

If that approach was already discussed, please let me know.

WDYT?

Thanks!