bug#15273: 24.3.50; Combining character sequences are displayed weirdly

[Eli Zaretskii <eliz@gnu.org>]

> From: Jan Djärv <jan.h.d@swipnet.se>
> Date: Sat, 7 Sep 2013 10:54:58 +0200
> Cc: xfq.free@gmail.com,
>  15273@debbugs.gnu.org
> 
> Except the larger bug that Emacs does not handle combining characters correctly in the general case, i.e. not combining the ! and the triangle on X11 and W32 (and Mac port).

Is there any application out there that does combine these 2
characters, with any font?  If not, perhaps it's not an Emacs issue
after all.

E.g., Code2000, which is really good, produces a composed character
for a followed by u+20d0, but not for a followed by u+20e4.  You can
experiment with other combining diacriticals from that Unicode block,
and you will see that some of them combine, while others do not.

If the fonts do not tell us to combine characters, the only way to do
that is to provide an Emacs composition rule for those characters.

> And if it is a bug in the NS font driver, that is also an Emacs issue.  There must be one, either metrics should be corrected or the NS font driver should be modified to behave in the same (incorrect) way as the other platforms.

If you mean the incorrect display of the lone u+20e4, then I just
installed the STIX fonts (from
http://sourceforge.net/projects/stixfonts/files/?source=navbar) on my
Windows box, and didn't see any problems as originally reported here.
Can you install the latest STIX fonts and try with that on NS?  If the
problem persists, then I agree that nsfont.m is probably the culprit.

> > But Xue now seems to say that there are other similar
> > situations with other combining characters, so we need to know if any
> > compositions are involved in those.
> 
> The exact key sequences that produced them would help also.

You can always use "C-x 8 RET" to type the characters, so I don't see
why the key sequences would be an issue.