M-x icomplete-mode
M-x set-variable icomplete-prospects-height 1
M-x a
(the candidates list takes up two lines in the minibuffer)

Here's a patch to make icomplete honor `icomplete-prospects-height'
again, which it hasn't since the introduction of `icomplete-separator'
(revno: 111031).

About formatting: the source appears to mix tabs and spaces for
indentation quite freely; I didn't attempt to fix this and went with
what emacs -Q did for formatting.

  * lisp/icomplete.el (icomplete-completions): Fix to honor
    `icomplete-prospects-height' again.