bug#38181: Actual height of mode-line not taken into account

[Eli Zaretskii]

> From: martin rudalics <rudalics@gmx.at>
> Date: Mon, 18 Oct 2021 19:44:08 +0200
> Cc: 38181@debbugs.gnu.org
> 
>  > I've tested it in both master and emacs-28 branches and I've seen none
>  > of the aforementioned issues nor, at least in a cursory examination,
>  > any regression.
>  >
>  > It seems like you're displaying the modeline instead of merely using
>  > the current height? There is no perceptible flickering in any case.
> 
> There's no "displaying" involved.

There is, for some value of "display": this function actually
generates a glyph row in the glyph matrix.

> Eli, I think this could go into Emacs 28.  WDYT?

No, it's too radical, and the problem it fixes is too obscure.  Let's
install it on master, please.