bug#53729: 29.0.50; Tamil text not shaped in modeline

[Visuwesh]

[வியாழன், பிப்ரவரி 03 2022] Eli Zaretskii wrote:

Hello, Eli

>> From: Visuwesh <visuweshm@gmail.com>
>> Cc: 53729@debbugs.gnu.org
>> Date: Thu, 03 Feb 2022 07:15:01 +0530
>> 
>> > on the mode line, the buffer is shown in bold, so perhaps there's a
>> > problem with the bold variant of the Noto Serif Tamil font?  If you
>> > type the same text in a buffer, but give it the 'bold' face, do you
>> > see the same problem with buffer text as on the mode line?
>> 
>> No, when I give it the bold face and insert it in a buffer, the text is
>> shaped properly.  Then, I tried setting mode-line-format to
>> "அக்கிச்சிறகுகள்" and '(:propertize "அக்கிச்சிறகுகள்" face bold) and in both
>> cases, the text is shaped properly.  I'm not sure where the problem is
>> anymore.  I attached screenshots of the same as well.
>
> Strange.  I guess the only way of investigating this is to step with
> GDB into the code which renders the mode line, and see which font
> specifically is being used there?
>

I suppose so but I'm confident that "Noto Serif Tamil" is the font used
in the modeline.  The only other Tamil font I have installed is "Noto
Sans Tamil" and I can easily make out the difference between the two.
Font selection does not seem to be the problem, at least.

In either case, I think I can only get to this in two weeks.  And is the
information in etc/DEBUG all I need (except the breakpoint which will be
provided?)?

> Btw, do I understand correctly that the problem you see is the
> incorrect location of the dot-like diacriticals above the letters?  Or
> is the problem something else?  (I don't read the Tamil script.)

You're right.  In the OP, even simple combinations like க + ் is not
rendered right: the dot should be on top of க but in the buffer name, it
is next to it.  However, Emacs seems to have no problem shaping க + ி.
The grossest of all is க + ு where the combined letter should be க plus
some kind of arc that surrounds the letter i.e., கு (hopefully Emacs
renders this fine on your end, if not, I guess I could write it down on
paper and send a picture).

I really hope the above explains the problems with shaping.