Re: Rendering performace vs. line-spacing

[Eli Zaretskii]

> Cc: emacs-devel@gnu.org
> From: "Herman, Geza" <geza.herman@gmail.com>
> Date: Fri, 8 Jan 2021 16:38:10 +0100
> 
> I disabled redrawing caused by overlapping by commenting out the line 
> which sets the "row->overlapping_p" flag (xdisp.c:21752), and the 
> problem is gone away. But my fonts are clipped (not a surprise). But, it 
> fixed the consolas case as well, and I don't see any clipping with this 
> font. Maybe overlapping detection is too strict (I mean, emacs considers 
> cases overlapping which are close to overlapping, but they aren't)? Or 
> maybe I just don't notice the clipping.

It could be that in the case of Consolas the overlapping doesn't
actually happen, but some metrics we get from the font tell us they
are.  Or maybe they do happen, but only when very few characters from
the font are used.  Or maybe this also depends on the font backend,
like whether you build with or without Cairo.

This calls for some font expert(s) to study the Consolas case and see
why the problem happens there.  Perhaps we could speed up at least
that case, I don't know.