bug#35721: 27.0.50; Strange Arabic shaping behavior

[Khaled Hosny <dr.khaled.hosny@gmail.com>]

On Thu, May 16, 2019 at 04:28:10PM +0300, Eli Zaretskii wrote:
> > From: Behdad Esfahbod <behdad@behdad.org>
> >
> > 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?

There are two kinds of formatting changes, that involve a different font
and that don’t.

For changes that involve a different font (using a new typeface, making
text bold or italic etc.) each part of the text on the sides of the
change has to be shaped separately, but if enough context is given then
HarfBuzz can at least do basic Arabic shaping (selecting the right form
for the context).

Changes that does not involve a different font like color, underline,
background etc. should not break the shaping at all and the text should
be shaped together. The formatting information should be kept along the
text and be applied after shaping i.e. if characters N to N+5 are red,
then after shaping glyphs belonging to characters N to N+5 should be
drawn red.

Regards,
Khaled