Bisecting between emacs-24.3 and emacs-24.4 led me to commit
680e6b8c5a28489733df544edb074fd29d0522a0, which introduces the
regression.  There is also a possibly related commit two hours later,
1269a680862a9bc6cd65e3d26ef05f68c7521632.

Commit 680e6b8c introduces the behavior observed in 24.4, where a frame
is not constrained to the screen area.  Also interesting is that
`display-pixel-width' works correctly (for multiple monitors as well,
that is) in the commit and the commits immediately preceding it, so it
may only be tangentially related.

I'm attaching the changeset of 680e6b8c and CCing the author.

I looked into constrainFrameRect, and it seems to have different
behavior depending on whether "Spaces" are available (introduced in OS X
10.9 apparently).  Might also have something to with the issue at hand.