Test was introduced in r225311.
Created attachment 327983 [details] Patch
Upon further investigation, the attachment element size seems to differ depending on whether a retina or non-retina display is used, which explains why we only encounter this on the Mac bots.
Comment on attachment 327983 [details] Patch Clearing flags on attachment: 327983 Committed r225332: <https://trac.webkit.org/changeset/225332>
All reviewed patches have been landed. Closing bug.
<rdar://problem/35776711>