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

[Visuwesh]

[ஞாயிறு, பிப்ரவரி 13 2022] Eli Zaretskii wrote:

Hi Eli,

>> From: Visuwesh <visuweshm@gmail.com>
>> Cc: 53729@debbugs.gnu.org
>> Date: Sun, 13 Feb 2022 20:26:17 +0530
>> 
>> >> and the buffer name in the modeline has the right shaping!  This is the
>> >> case for _every_ font I tried: including Noto Serif Tamil.  But if I
>> >> open a file named "அக்னிச்சிறகுகள்.pdf", the shaping is as in the OP.
>> >
>> > What happens if you turn off auto-composition mode before repeating
>> > the above experiment?  Do you see any difference in the buffer name
>> > produced by you and the buffer name produced by Emacs when visiting
>> > that file?
>> 
>> If I turn off global-auto-composition-mode and do the above, none of the
>> text is shaped i.e., the buffer name produced by me and the buffer name
>> produced by Emacs both are not shaped.
>
> Of course they aren't shaped: turning off auto-composition-mode
> disables the shaping.  I'm asking whether both the buffer name
> produced by you and the buffer name produced by visiting that file
> look identical on the mode line, or do they somehow differ?  If they
> do differ, what is the difference?
>

I misunderstood what you meant, sorry.  They look the same.

>> I did this out of curiosity: in dired, I typed C M-n when over the file
>> and added ".1" to the end of new file name, and when I visit this file,
>> the buffer name is shaped properly.  [ The new filename is
>> அக்னிச்சிறகுகள்.pdf.1 ]
>
> So you are saying that changing the file-name extension affects the
> shaping on the mode line?
>

Yes, that seems to be the case.  I tried changing the extension to
"jpeg", and the shaping was incorrect.  If I completely remove the
extension, the text is shaped properly.

>> I'm not sure if this has to do with the filename since when I yank the
>> file name from dired and create an empty file (M-x
>> dired-create-empty-file) by that name in another directory and visit it,
>> Emacs shapes the buffer name properly.
>
> Very strange.  Is that the only file name with such problems?  It
> sounds like maybe its file name has more than meets the eye (which is
> one reason why I asked you to disable auto-composition-mode).

I'm not sure if that's the case.  If I rename that file in Emacs to
"வணக்கம்.pdf", the text is not shaped properly again.

Also, I found out that non-empty files (with Tamil names) don't have
their buffer name shaped properly.  If I create a new empty file in
dired, the buffer name is shaped properly but that is not the case if I
rename an existing, non-empty file.