Certain configurations of hardware accelerated content hit this assertion in RenderView. It indicates that RenderObject::container() returned an object above repaintContainer in the hierarchy. It's triggered by compositing layers made for opacity fades in between position: absolute layers.
<rdar://problem/7032254>
Created attachment 40155 [details] Patch
http://trac.webkit.org/changeset/48816
This fix doesn't get all cases.
*** Bug 28898 has been marked as a duplicate of this bug. ***
Created attachment 40335 [details] Better fix
Comment on attachment 40155 [details] Patch Obsoleting this patch that looks like it was already landed.
http://trac.webkit.org/changeset/48932