Looks like the fix from bug 139884 are actually needed on Mavericks too. I previously limited it to newer OS versions, because it seemed like Mavericks was not affected, but it is.
For example, fast/css/object-fit/object-fit-canvas.html gets 1600x1200 results, and actually non-matching ones.
Created attachment 248529 [details]
Comment on attachment 248529 [details]
Clearing flags on attachment: 248529
Committed r181451: <http://trac.webkit.org/changeset/181451>
All reviewed patches have been landed. Closing bug.