bug#35721: 27.0.50; Strange Arabic shaping behavior

["Basil L. Contovounesios" <contovob@tcd.ie>]

Eli Zaretskii <eliz@gnu.org> writes:

> tags 35721 notabug
> thanks
>
>> Date: Thu, 16 May 2019 16:20:00 +0200
>> From: Khaled Hosny <dr.khaled.hosny@gmail.com>
>> Cc: behdad@behdad.org, contovob@tcd.ie, handa@gnu.org,
>> 	35721@debbugs.gnu.org
>> 
>> On Thu, May 16, 2019 at 05:08:47PM +0300, Eli Zaretskii wrote:
>> > The main point of this bug report is about the other part, though: how
>> > sheen should be displayed when using DejaVu Sans Mono.
>> 
>> I don’t think there is a right answer here. Glyphs can have -ve right or
>> left side bearings, so if the graphics context has no room on the either
>> side then I think it is expected that the glyph would be truncated. This
>> is not Arabic specific (e.g. italic f often have -ve right side
>> bearing, in variable width fonts at least).
>
> OK, thanks.
>
> Basil, is it okay with you to close this bug, given these comments?

Thanks to everyone for your explanations.  I now understand why the
truncation happens, and why mark activation is expected to affect
character composition.

The issue that prompted this report, however, is the alternating
toggling of certain character compositions while typing in a separate
part of the buffer (steps 9 and 10 in the OP), where no face change is
involved.  I'm sorry for not making this clearer from the outset.

If you think this is unsurprising behaviour given the display engine's
current implementation, I don't mind if you close this issue.
Otherwise, perhaps either this issue can be retitled, or I can submit a
new issue focussing only on that part of the OP.

Sorry for the confusion, and thanks again.

-- 
Basil