bug#11860: 24.1; Arabic - Harakat (diacritics, short vowels) don't appear

[Kenichi Handa]

In article <wlbohflu0z.wl%mituharu@math.s.chiba-u.ac.jp>, YAMAMOTO Mitsuharu 
<mituharu@math.s.chiba-u.ac.jp> writes:

> As you explained, the grapheme clusters are in logical order, and
> glyphs in each grapheme cluster are in drawing order.  So simply
> merging grapheme clusters as in the code in Ffont_shape_gstring does
> not seem to be correct in the case of right-to-left text (what's drawn
> later comes earlier in a merged grapheme cluster).

Sure.

> IMO, dividing glyphs into grapheme clusters is font backed driver's
> task, and I don't understand why Ffont_shape_gstring merges the
> grapheme clusters for some cases.  Could you explain?

When I designed it, I consider such a situation that
grapheme clusters returned by a font-driver is so fine for
Emacs' display engine that Ffont_shape_gstring must combine
some of them into one grapheme cluster.  But, I agree that
it's much cleaner to make a font-driver to consider such a
thing.

I'll try to fix Ffont_shape_gstring, and check whether or
not it breaks rendersing of some scripts.

---
Kenichi Handa
handa@gnu.org