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

[Eli Zaretskii]

> From: Eli Zaretskii <eliz@gnu.org>
> Date: Tue, 22 Sep 2020 01:26:49 -0400
> Cc: 43519@debbugs.gnu.org
> 
> Actually, we can do better.  I will soon install a simple change that
> fixes the problems discussed in this bug and in all the related
> discussions I'm aware of.  We can then take it from there, if some
> issues remain.

Done.

I will wait for a week or two for reports about things this change
breaks, or about issues still left to be fixed after this commit, and
if nothing surfaces, I will close the bug report.

Thanks.