bug#20140: 24.4; M17n shaper output rejected

[Richard Wordingham via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On Tue, 15 Feb 2022 16:40:09 +0200
Eli Zaretskii <eliz@gnu.org> wrote:

> > Date: Mon, 14 Feb 2022 23:26:23 +0000
> > From: Richard Wordingham <richard.wordingham@ntlworld.com>
> > Cc: larsi@gnus.org, 20140@debbugs.gnu.org
> >   
> > > No, that's not true.  I'm not aware of any such limitation; AFAIK
> > > Arabic shaping works correctly in Emacs, certainly with HarfBuzz
> > > and Emacs 27 or later.
> > > 
> > > Or maybe I misunderstand what you mean by "typewriter-like" fonts?
> > > Can you give an example of a non-typewriter-like font for Arabic
> > > that I can find on MS-Windows and try?  
> > 
> > Not off the top of my head, but compare لحج with the presentation
> > form ‎ﳊ U+FCCA ARABIC LIGATURE LAM WITH HAH INITIAL FORM for the
> > first two letters.  The lam part is a vertical line in the middle
> > of the glyph; the 'hah' part forms the lower part of the glyph.  
> 
> They look identical here (using the default Courier New font).  With
> what font did you think they will look wrong?

In the Courier New font in Windows 10 of 2017 (+ automatic updates),
U+FCCA looks like the image in the Unicode code chart, and bears little
resemblance to the righthand two thirds of <U+0644, U+062D, U+062C>.
In keeping with its Latin part, the sequence of three characters looks
as one would expect from a typewriter when one enters text letter by
letter.  I must admit I'm having trouble laying my hand on a font which
does these ligatures.  I wanted to find a font that would render the
three characters to look the same as ﳊﺞ <U+FCCA, U+FE9E>.  (Sticking
them together isn't working in the email client I'm using, but does
work in some fallback font.)

Richard.