bug#17261: Some combining characters don't combine (Emacs 24 regression)

[Eli Zaretskii]

> Date: Sun, 13 Apr 2014 10:49:44 -0500
> From: Max Shinn <max@maxshinnpotential.com>
> 
> Certain combining characters---such as the combining vector
> arrow (0x20D7, a⃗) and the combining circle (0x20DD, a⃝)---do not
> combine properly under most fonts on Emacs 24.3.1 under Debian GNU/Linux
> x86_64, GTK 3.4.2.  In Emacs 23.3.1, these characters worked properly
> under several fonts (for instance, Droid Sans Mono), whereas in Emacs
> 24, they only work properly with the FreeMono font.  The precise
> behavior I am observing on Emacs 24 is where the base character and the
> combining character sit side-by-side instead of superimposed on one
> another.

Looks like some side effect of changes in how we select fonts.  Where
Emacs 23 would force the base character to use the font that could
display the combining character, Emacs 24 now does not, which disables
auto-composition because Emacs can only compose characters that come
from the same font.  So the only way to display those characters as
composed is to select a buffer font that can display them.