bug#28339: 25.2; Emacs shows ZWNJ character (Zero Width non-Joiner) as Space

[Eli Zaretskii <eliz@gnu.org>]

> From: handa <handa@gnu.org>
> Cc: b.riefenstahl@turtle-trading.net, nimawebgard@gmail.com, 28339@debbugs.gnu.org
> Date: Mon, 18 Sep 2017 10:52:41 +0900
> 
> > > I thought that with OpenType at least that depends on the font?
> 
> > It does, but Handa-san seems to say that even the best fonts don't
> > consider ZWNJ part of any grapheme cluster, and always leave it alone.
> 
> I checked the GSUB table of "Courier New" font (cour.ttf) using the
> program ttx (included in fonttools package of Ubuntu).  It surely
> contains many rules with ZWNJ, but none of them are with Arabic
> characters.  So, I suspect that absorbing of ZWNJ for Arabic is done by
> a layout engine (halfbuzz? uniscribe?) or by an application level
> library (pango?).
> 
> > > For editing, I would like a hair-line type glyph to delete.
> 
> > We already have a solution for deleting a character which was composed
> > with the preceding one(s).  So I think this aspects doesn't have to be
> > a factor in our decision how to display ZWNJ.
> 
> Isn't there a case that ZWNJ is prepeneded to a character to change the
> shape of the following character?

I don't see this in Unicode, but maybe I'm missing something.

Anyway, what would you suggest as a solution to this issue?  Should we
install the arabic-font-shape-gstring function into Emacs?  Do we need
to do something else in addition?  E.g., do we need to make the
display of ZWNJ optional?

Thanks.