Attached testcase shows an issue with the placement of the box contents with overflow. Hovering over the white box should not cause the contents to jump around.
The contents are moving by the border with, so this is an issue with the geometry of the foreground layer.
Created attachment 34944 [details] Patch
Comment on attachment 34944 [details] Patch r=me
Comment on attachment 34944 [details] Patch Leaving this for smfr to commit himself. He can change it to cq+ if he'd prefer the bot to land it for him.
http://trac.webkit.org/changeset/47409