bug#44784: Combining accent is not combining

[Eli Zaretskii <eliz@gnu.org>]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: Juri Linkov <juri@linkov.net>,  44784@debbugs.gnu.org
> Date: Sun, 22 Nov 2020 11:12:03 +0100
> 
> > The 2 vectors you show above, let alone the "composed with" part,
> > explicitly say they _were_ composed.  So something else is going on
> > here.
> 
> I think it's a display problem with DejaVu Sans Mono, as you already
> noted.  I also see what Juri sees, but when the cursor appears to be
> over the separately displayed accent, `C-u C-x =' says the character at
> point is SPC.

Right.  I think the discussion on the HarfBuzz list to which I pointed
explains what happens: the problematic fonts don't provide the
information necessary to correctly locate the accent over some
character (specifically, precomposed characters that are already
accented).  The [0 0 0] part of the second glyph in the composition
info is a telltale sign of lack of that information: if you try the
same with a font where display is correct, you will see that the first
2 components are non-zero (notably, the first component should be
negative).