bug#43519: 28.0.50; Overlay at end of minibuf hides minibuf's real content

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Sat, 19 Sep 2020 13:54:13 -0400
> 
>     % src/emacs -Q --eval '(setq max-mini-window-height 1)' -f icomplete-mode
>     M-x a
> 
> at this point, you should presumably not see the "M-x a" in your
> minibuffer window but only something of the form "{rp | lign | ..."

Seems like a bug in icomplete: it attempts to compute the maximum
length of candidates to be displayed, but seems like it fails, because
the single mini-window line is continued, with no ellipsis at the end
of the visible line?

The fact that it calls window-width with no arguments is one possible
problem -- it assumes the default face's font.  But I think the
problem is more prominent than just that.

It should produce an overlay string that fits in the window, then the
prompt will be visible.

Am I missing something?