bug#24293: 25.1; Display bug: icomplete prompt not visible with icomplete-separator "\n"

[Dmitry Gutov]

FWIW, somebody published a nicely-working icomplete-vertical-mode recently:

https://github.com/oantolin/icomplete-vertical

According to the author, it deals with this particular problem with a 
combination of (setq truncate-lines t) and an enlarge-window call: 
https://www.reddit.com/r/emacs/comments/fswt7c/using_icomplete_vertically/fm8z6h0/

The only downside to this mode I could find is that it keeps the 
minibuffer fixed height (the max prospects count plus 1). But it can be 
considered an upside as well, matter of taste.