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

[Richard Wordingham <richard.wordingham@ntlworld.com>]

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.

Richard.