bug#42256: 27.0.50; composition

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: rms@gnu.org,  42256@debbugs.gnu.org
> Date: Tue, 14 Jul 2020 03:13:12 +0300
> 
> I tried, but got a different output:

The output you get depends on the font as well, so I see no problem
here.

> > And at position 872 you can see an even more stark example: instead of
> > 2 characters, we show 8(!).
> 
> I don't understand where these 8 characters are coming from.
> This composition of 8 characters is returned by find-composition.
> Maybe the bug is in find-composition?

No, there's no bug in find-composition: it returns what we should pass
to the text shaper.  The problem here is that your code assumes all
the characters we passed to the shaper are a single grapheme cluster,
which is not true.

I suggest to look at the code which displays the "Composed with" line
and decides which characters to show there, and do the same in your
addition.

Thanks.