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

[Eli Zaretskii]

> From: Andreas Schwab <schwab@linux-m68k.org>
> Cc: Jan Djärv <jan.h.d@swipnet.se>,  xfq.free@gmail.com,
>   15273@debbugs.gnu.org
> Date: Fri, 06 Sep 2013 08:42:23 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Character composition in Emacs can happen in 1 of 2 ways:
> >
> >  . The font driver tells Emacs to compose several characters into a
> >    single grapheme cluster, by drawing all of them as a single unit,
> >    and by drawing the 2nd, 3rd, etc. character glyphs at certain pixel
> >    offsets relative to the base glyph.
> >
> >  . Emacs itself has composition rules for 2 or more characters; in
> >    this case, the same pixel offsets come from those rules.
> 
> The first way can only work if both characters are coming from the same
> font.

Yes.

> Not sure if that is also true for the second way, but I'd guess yes.

I'm not sure, either.  The second way happens when we generate the
glyph matrices -- do we select font before or after that?  If after,
we probably look for a font that supports all of the characters in the
composition.  Perhaps Handa-san could comment on this.

But, at least in my testing, I used a font that definitely supports
both characters, and the composition still didn't happen.