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:
-- Anders