bug#54646: 29.0.50; set-fontset-font and font clipping issues

[Eli Zaretskii]

> From: Visuwesh <visuweshm@gmail.com>
> Cc: 54646@debbugs.gnu.org
> Date: Thu, 31 Mar 2022 14:15:41 +0530
> 
> I have attached three text files that have the content of the *Help*
> buffer in the three cases:
> 
>     · correct: from emacs -Q which does not exhibit the problem.
> 
>     · incorrect: from an non-daemon Emacs session that exhibits the
>       problem.
> 
>     · correct_config: from a fresh Emacs session with my init.el loaded
>       that does not exhibit the problem.

For meaningful comparison, I need data for the same font size.  One of
the three samples uses a smaller font size, so it's hard to compare it
to the rest.

However, this:

> Composed with the following character(s) "்" using this font:
>   ftcrhb:-Goss-Kurinto Seri-regular-normal-normal-*-14-*-*-*-*-0-iso10646-1
> by these glyphs:
>   [0 1 2965 23479 12 0 12 8 0 nil]
>   [0 1 3021 23505 0 -1 1 11 -9 [-6 0 0]]
> with these character(s):
>   ் (#xbcd) TAMIL SIGN VIRAMA

vs this:

> Composed with the following character(s) "்" using this font:
>   ftcrhb:-Goss-Kurinto Seri-regular-normal-normal-*-14-*-*-*-*-0-iso10646-1
> by these glyphs:
>   [0 1 2965 23479 12 0 12 8 0 [0 0 20]]
>   [0 1 3021 23505 0 -1 1 11 -9 [-14 0 0]]
> with these character(s):
>   ் (#xbcd) TAMIL SIGN VIRAMA

Seems to indicate that we use incorrect composition data in the second
case: the X offset part (-14) seems to be too large, and I don't
understand why there's a non-zero WADJUST value (20) for the first
glyph of the grapheme cluster.  Very strange.  Are both cases for
exactly the same text that surrounds the problematic characters, or
does the surrounding text differ in any way?

Also, what version of HarfBuzz do you have there?  Can you try
upgrading to a newer version?