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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 27 Mar 2015 09:04:44 +0000
> From: Richard Wordingham <richard.wordingham@ntlworld.com>
> Cc: 20173@debbugs.gnu.org
> 
> On Tue, 24 Mar 2015 19:03:38 +0200
> Eli Zaretskii <eliz@gnu.org> wrote:
> 
> > > Date: Tue, 24 Mar 2015 08:28:28 +0000
> > > From: Richard Wordingham <richard.wordingham@ntlworld.com>
> > > Cc: 20173@debbugs.gnu.org
> 
> > > You might want to first check whether composed Arabic is
> > > usable. Doesn't making each word a grapheme cluster makes editing
> > > unpleasant?
> 
> > I don't know; I don't speak or write any of the languages that use the
> > Arabic script.  I expect the users that do to come up and ask for
> > features they miss.  We already allow deletion of single codepoints,
> > even when they are composed; we might as well provide similar features
> > for movement or whatever.
> 
> I forgot that grapheme clustering is done in m17n, not Emacs itself.
> The m17n code (in ARAB-OTF.flt) is reasonable - it clusters letters
> with combining marks.  It *seems* I have a problem with tpu-forward-char
> and tpu-backward-char; it's as though there's an initialisation fault
> which stops them stepping through the Arabic compositions at first.  It
> may be an issue with the presumably underlying forward-char and
> backward-char; I haven't investigated further.  I'll have to record
> the exact actions provoking the problem before I formally record a bug.

Please try in "emacs -Q" without activating the TPU emulation.