bug#39390: 28.0.50; prettify-symbols-mode confuses display when start/end faces are different

[Eli Zaretskii]

> Cc: 39390@debbugs.gnu.org
> From: Raphael 'kena' Poss <knz@thaumogen.net>
> Date: Sun, 2 Feb 2020 21:00:53 +0100
> 
> If you are telling me that (compose-region) should never have agreed to
> operate on a range of symbols with different faces, then I have
> counter-evidence that it does. If you are telling me that it shouldn't,
> then maybe that's where the bug really lies.

I'm telling that it cannot work well in these circumstances, and you
shouldn't expect it to.