overflow:scroll should not leave space for a scroll corner on the Mac with overlay scrollbars because it looks wrong. Historically, overflow:scroll elements were meant to have scrollbar tracks and scroll corners whether or not there was content to scroll to. Since overlay scrollbars only appear when the user is scrolling, we don't do anything to differentiate overflow:scroll from overflow:auto if there is nothing to scroll too -- EXCEPT that we leave space for a scroll corner, which just looks wrong. We should stop doing that and embrace the fact that overflow:scroll just means something slightly different in the world of overlay scrollbars.
Created attachment 238191 [details]
Here's a test case that shows the problem. It's silly that we leave space for the scroll corner in the top two <div>s. It only makes sense in the bottom <div>.
Created attachment 238192 [details]
Thanks Sam! http://trac.webkit.org/changeset/173668