This is regarding Emacs.app, built with --with-ns from GNU emacs 24.1.1
source.

Pre-composed characters (e.g. 0xf1, ñ) render perfectly, but combining
diacritics (e.g. n\x303) render the character oddly. Namely, the diacritic
very miniscule, and is not placed directly above the main character (but
not entirely to the left or right either). However, a single glyph IS
created, since C-f and C-b skip over the entire rendered character.

Oddly, dired correctly displays both pre-composed and de-composed
characters, but other modes do not (org, shell, text modes, GNU's python.el
inferior shell).

Of course, either method works fine in other appliations (Chrome,
Terminal.app, LibreOffice, native OS X apps). Further, both methods work
when running in non-windowed mode in Terminal.app.

Note that this is not precisely the problem described here (
http://www.emacswiki.org/emacs/CarbonEmacsPackage#toc23)  In particular,
dired works fine, non-windowed mode works fine, and in other modes, the
main character and the diacritic ARE being composed, it's just the final
glyph is rendered oddly.

Dan