bug#44632: 27.1.50; Left-to-right Arabic shaping breaks at property bounds

[Stefan Kangas]

tags 44632 + wontfix
close 44632
thanks

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Thamer Mahmoud <thamer.mahmoud@gmail.com>
>> Date: Sat, 14 Nov 2020 15:42:35 +0300
>>
>> (with-current-buffer-window "*shaping testcase*" nil nil
>>   (insert "L2R عربي" "\n\n")
>>   (insert "عربي")
>>   (put-text-property 5 6 'face 'highlight)
>>   (put-text-property 11 12 'face 'highlight))
>>
>> 2. Notice the AIN(ع) in the L2R paragraph has the wrong shape.
>
> This is a known limitation of the current Emacs display engine: it
> cannot shape text across face changes.  All the text that needs to be
> shaped must have the same face.  This problem has existed since Emacs
> 21.
>
> Fixing this limitation would require a more or less complete redesign
> of how the display engine scans the buffer text, or maybe some radical
> changes in how character composition is handled.  Until one of these
> changes, this problem will remain unsolved, sorry.

Since this is a known limitation, I think it unfortunately doesn't make
sense to keep this bug open.  If anyone wants to work on a redesign of
the display engine, they will probably not do that due to this bug.

I guess it would be good if we could have a list somewhere of the known
limitations, so if anyone knows about something like that please add it.

But for now, I'm closing this as wontfix.