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

[Visuwesh]

[வியாழன் மார்ச் 31, 2022] Eli Zaretskii wrote:

>> From: Visuwesh <visuweshm@gmail.com>
>> Cc: 54646@debbugs.gnu.org
>> Date: Thu, 31 Mar 2022 19:14:23 +0530
>> 
>> I compiled an Emacs that is linked against HarfBuzz 3.3.2, and it shows
>> the same problem.
>
> The latest version of HarfBuzz is 4.2.0, although the chances that
> it's their problem are not great.
>
>> I managed to reproduce this in an emacs -Q session with that evaled but
>> it took me some time [*].  For each buffer that has Tamil text, I have
>> to zoom in (or none in the case of eww's header-line) different amounts
>> to see the clipping issue:
>> 
>>     · In init.el, I have to zoom in 7x times
>>     · In eww, I have to zoom in 1x time
>>     · In dired, I have to zoom in ~15x times
>> 
>> In the problematic emacs -Q session, current-iso639-language is 'en'.
>> 
>> [*] As in, it literally took me half an hour to observe the issue, and I
>>     cannot seem to figure out a quick way to reproduce it.
>
> Can you provide a recipe, as in: what file or URL to visit and/or what
> text to insert before starting the zoom commands?  Also, do you zoom
> in and out repeatedly, or just zoom in that many time?  And if it just
> takes a small number of zoom commands, why do you say it takes half an
> hour? what else needs to happen during that time to see the
> problematic display?
>

It looks like I was mistaken about the time it takes to reproduce; I was
simply not observant enough.  Here's a recipe that reliably reproduces
the problem,

    1. emacs -Q
    2. M-: (set-fontset-font t 'tamil "Kurinto Seri")
    3. M-s M-w https://www.dinamalar.com/news_detail.asp?id=2996410
    4. C-x C-+ a bunch of times and look for clipped text.  Sometimes
       this does not reproduce the first time, so you end up having to
       zoom out and in a few times.

> Thanks.

>From the other mail:
> One more question: does this happen only with that particular font
> used for Tamil?

I tried Meera Inmai, Noto Serif Tamil, Noto Sans Tamil, Catamaran,
Kurinto Seri.  Out of which only Noto Serif and Kurinto Seri shows the
problem.  Maybe it is a font issue after all? but (Ungoogled) Chromium
which uses harfbuzz AFAICT renders the text just fine.