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

[Visuwesh]

[வியாழன், பிப்ரவரி 03 2022] Eli Zaretskii wrote:

>> 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.
>
> It could be that Emacs selects some variant of Noto Serif Tamil (some
> weight or maybe width) which causes this.

Right.  I know I cannot trust my eyes but it seems to be the bold font
but I will step through in gdb.

> Btw, can you try this with other fonts and see if any of them displays
> the buffer name correctly in the mode line?
>

I tried "Lohit Tamil" but the text is not shaped properly when I use it
too, and the (incorrect) shaping is different from what I observe when I
use "Noto Serif Tamil".  I will try other fonts over the weekend and
report back.

>> 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?)?
>
> It should get you started, yes.  There are special commands defined in
> src/.gdbinit that will help showing Lisp objects, and feel free to ask
> for guidance if you aren't sure how to proceed or have any questions.
>

Thanks.

>> 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).
>
> OK, thanks.
>
> One more question: which version of HarfBuzz do you have installed
> there?

HarfBuzz 3.0.0, and if it matters, I have: Cairo 1.16.0, Pango 1.48.10.