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

[Eli Zaretskii <eliz@gnu.org>]

> From: Visuwesh <visuweshm@gmail.com>
> Cc: 54646@debbugs.gnu.org
> Date: Thu, 31 Mar 2022 17:46:51 +0530
> 
> [வியாழன் மார்ச் 31, 2022] Eli Zaretskii wrote:
> 
> >> From: Visuwesh <visuweshm@gmail.com>
> >> Cc: 54646@debbugs.gnu.org
> >> Date: Thu, 31 Mar 2022 14:59:47 +0530
> >> 
> >> > Also, what version of HarfBuzz do you have there?  Can you try
> >> > upgrading to a newer version?
> >> 
> >> Emacs is linked against HarfBuzz 3.0.0.  I will see if I can update it,
> >> and report back in the evening.  (P.S., maybe we should look into
> >> including this info in the text that M-x report-emacs-bug prepares?)
> >
> > HarfBuzz is remarkably compatible, and its version until now was never
> > important.  I asked about that because it is the source of the
> > composition data which seems to be incorrect in the wrong display
> > cases.  I'm not yet sure it's a HarfBuzz problem.
> >
> >> [ I will also check if I can reproduce this in emacs -Q but with 
> >>   M-: (set-fontset-font t 'tamil "Kurinto Seri") RET.  ]
> >
> > Thanks.
> >
> > One more question: what is the value of current-iso639-language, and
> > is it different between the "bad" and the "good" cases?
> 
> 'ta' in both case.

Thanks.  Then I'm out of ideas, I'm afraid.  The data comes from
HarfBuzz, so if it's our fault, we must feed it something differently
in each case, and I cannot see what that could be...

> BTW, the "bad" case having width 20 seems "correct" to me since the
> character occupies more space than it should i.e., imagine a letter like
> "I" but with a bunch of whitespace next to it as in "I  ".

That 20 and the larger value of X offset are the only differences
between the "bad" and the "good" cases, so they must be the
explanation.