Eli Zaretskii <eliz@gnu.org> writes:

> Can you show a screenshot?

Attached.

> Something with your fonts and fontsets, I guess.  If you force 
> Emacs to use Inconsolata-g for "̷", does the composition happen?

Hmm, i'm having some difficulty doing this. i start GUI Emacs from 
an X terminal via 'emacs -Q'. i then go to the Options menu, 
select "Set Default Font", and choose "InconsolataG Medium"[1]. In 
*scratch*, i type 'o', and confirm with `describe-char' that 
"InconsolataG" is the font used for that character.

i then evaluate:

    (set-fontset-font "fontset-default" '(#x0337 . #x0337) 
    "InconsolataG")

with C-x C-e, which returns nil. If i then move point immediately 
after the previously-typed 'o', and type C-x 8 RET 337, the '/' is 
displayed visually separate from the 'o'. But using 
`describe-char' on it says that Gentium is the font used.

i gather something i'm doing something wrong here, i'm guessing in 
the ELisp .... ?

[1] "InconsolataG" is "Inconsolata-g" renamed by me locally, via 
FontForge, so as to be XLFD-friendly.