bug#20173: 24.4; Rendering misallocates combining marks on ligatures

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 23 Mar 2015 22:41:07 +0000
> From: Richard Wordingham <richard.wordingham@ntlworld.com>
> Cc: 20173@debbugs.gnu.org
> 
> On Mon, 23 Mar 2015 17:38:52 +0200
> Eli Zaretskii <eliz@gnu.org> wrote:
> 
> > > Date: Mon, 23 Mar 2015 01:06:26 +0000
> > > From: Richard Wordingham <richard.wordingham@ntlworld.com>
> 
> > Is it possible that some rule(s) are missing from the end of
> > lisp/language/misc-lang.el?  Could you please take a look and see if
> > something needs to be fixed/added in how we set up the compositions
> > for Arabic?
> 
> There's no relevant problem there.  I demonstrated the bug to myself by
> first rendering Tai Tham <NA, TONE-2, SIGN AA> and confirming that
> TONE-2 rendered above the first component of the ligature NAA, fromed
> from <NA, SIGN AA>.  I then hacked my font so that the glyph for TONE-2
> was decomposed into the glyphs for MAI KANG and TONE-2, in that order,
> and observing TONE-2 being rendered on the second component of the
> ligature.  I then turned to Arabic so that a custom font would not be
> needed to demonstrate the bug.

Sorry, I'm not sure I understand you.  If the setting of composition
rules for Arabic is not the culprit, then what is?  AFAIK, there are
no rules that guide Emacs's shaping except what's in
composition-function-table.  Beyond that, the only other factor is the
font backend and how it shapes glyphs given the chunks of text Emacs
presents to it.

> As to what needs fixing in the Arabic section of misc-lang.el:

Thanks, I will look into these.