bug#34035: 26.1; Arabic shadda-kasrah renders incorrectly

[Stephen Berman]

On Thu, 10 Jan 2019 20:55:55 +0200 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: "Peter" <craven@gmx.net>
>> Date: Thu, 10 Jan 2019 18:20:32 +0100
>> 
>> - Start emacs -Q
>> - C-\ arabic RET
>> - s ~ A (sin shadda kasrah) repeat a few times
>> 
>> The kasrah (short slash below the letter) should be shown below the
>> shadda (the squiggle above the letter) and above the letter, not below
>> it.
>> 
>> I've tried different fonts, this seems to be a problem with emacs
>> itself, other editors and firefox render the same text (copy-pasted)
>> correctly.
>> 
>> See attached screenshot for what it looks like in Emacs and in Firefox.
>
> Thanks.  On my system, this is displayed correctly, with kasrah above
> the letter.  So I don't think it's Emacs, I think it's the shaping
> engine you are using.  As you didn't provide the information collected
> by "M-x report-emacs-bug", I can only guess what is that shaping
> engine: XFT and libflt, right?  Maybe you could try building the
> harfbazz branch, which uses HarfBazz for shaping, I'd expect this
> problem not to exist there.

I believe the problem is not with the shaping engine but with the font:
I see the same problem on both builds from current master (with libotf)
and from the current harfbuzz branch using my default font, DejaVu Sans
Mono.  But when I switch the font to Symbola, the kasrah is correctly
displayed between the sin and the shadda, both on master and on
harfbuzz.  (Nevertheless, on both branches, after switching to Symbola,
describe-char surprisingly says this:

Composed with the following character(s) "ِّ" using this font:
  xft:-PfEd-DejaVu Sans-normal-normal-semicondensed-*-15-*-*-*-*-0-iso10646-1

Is this expected?)

Steve Berman