Re: how reliable is rendering of complex scripts?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 05 Oct 2015 22:22:15 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: emacs-devel@gnu.org
> 
> > The ZWNJ seems to work fine, but the ZWJ does not produce the behavior
> > documented here:
> > 
> >     http://persian.nmelrc.org/persianword/zwj.htm
> > 
> > That is, ZWJ does not extend the heh into a "heh do chesm" (heh with two
> > eyes), it just adds space before or after the final-form (circular) heh.
> 
> I tried to fix that, but I'm not sure it helped.  Maybe I simply don't
> understand how to write entries for composition-function-table.
> Perhaps Handa-san could help.

I found a subtle bug in the Emacs bidi reordering engine that affected
this.  After fixing it, the display is more reasonable in these cases.
The only remaining problem is display of sequences where the
zero-width controls are not between 2 Arabic letters, e.g. a u+0647
followed by ZWJ and a newline, in a left-to-right paragraph, a
relatively rare situation (it does work correctly in R2L paragraphs).
The special requirements for treating ZWJ and ZWNJ during the bidi
reordering are hard to implement, and the advice in the UBA is
incompatible with the design of the Emacs display engine.  I will have
to think about this some more.

One thing that still bothers me is that even though the shaping of the
Arabic letters is clearly affected by ZWJ and ZWNJ after the fix, the
"C-u C-x =" command doesn't say there was a character composition
there.  Perhaps Handa-san could explain why is that.  Is it a bug?