bug#35721: 27.0.50; Strange Arabic shaping behavior

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 16 May 2019 15:45:08 +0200
> From: Khaled Hosny <dr.khaled.hosny@gmail.com>
> Cc: Behdad Esfahbod <behdad@behdad.org>, contovob@tcd.ie, handa@gnu.org,
> 	35721@debbugs.gnu.org
> 
> > 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.

Right, thanks.

The main point of this bug report is about the other part, though: how
sheen should be displayed when using DejaVu Sans Mono.