bug#35721: 27.0.50; Strange Arabic shaping behavior

[Eli Zaretskii <eliz@gnu.org>]

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> From: Behdad Esfahbod <behdad@behdad.org>
> Date: Wed, 15 May 2019 16:02:19 -0700
> Cc: contovob@tcd.ie, Kenichi Handa <handa@gnu.org>, 
> 	Khaled Hosny <dr.khaled.hosny@gmail.com>, 35721@debbugs.gnu.org
> 
> Pretty much things you said: if font is not mono-spaced, there's nothing we can do.

Thanks.  Let me be sure I understand what you are saying.  If I invoke
hb-view like this:

  hb-view -u 0x0634 -O png -o sheen.png DejaVuSansMono.ttf

then the result is the following PNG image:


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Do I understand you correctly that this is the expected result with
that font?  Because what Emacs displays, even without HarfBuzz as its
shaping engine, looks exactly like that: the leftmost part of the
letter is off-screen.

> Also, if you don't pass neighboring context text to HarfBuzz, again,
> nothing we can do.

I believe this is about the other part: displaying text which is
partially selected, when selection is shown as a different background
color.  You are saying that to do its job, a shaping engine needs to
see the entire text, not just the part which has the same colors.
Right?

Thanks again for your comments.