This problem has been fixed and published on "master" and backported to
"emacs-25" (after being approved by Eli).

In the end it was not related to OS X. Instead, it was a function in
window.el used to by terminal windows to track the window width that didn't
take the continuation glyph into account. (The continuation glyph is
visible when the width of a fringe is zero.)

See the following for details:

http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=5b5403289888efe8783ae6a405845b925f544ec1

http://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-25&id=ff7e201ed87d206334afedd4366deebba440efde

    -- Anders