On the URL (and any YouTube video), iff accelerated compositing is enabled, when hovering over any thumbnail in the related videos section, a transition occurs. During the transition, all other thumbnails’ white horizontal borders vanish, revealing more of the thumbnail.
<rdar://problem/8088165>
The thumbnails use clip: style, and hardware clipping doesn't support this yet.
Created attachment 58615 [details] Testcase
Created attachment 58674 [details] Patch
http://trac.webkit.org/changeset/61200