bug#39433: Small values of ido-max-window-height hide the prompt when there are enough completions

[Dmitry Gutov]

https://github.com/creichert/ido-vertical-mode.el/issues/48#issuecomment-582202881 
says:

emacs -Q
M-x ido-mode
M-: (setq ido-max-window-height 1)

Then try C-x C-f in a directory with a significant number of files (> 
10, let's say, although that depends on their names' lengths).

The prompt and whatever you're typing won't be visible until you narrow 
the completions list down.

(Seems like another display engine peculiarity related to bug#39379).