We have logic that detects when an iframe is overlapped, and then calls -setCopiesOnScroll:NO on the iframe's content view so that AppKit does a full repaint of the view when it gets scrolled. However, this logic only works for a single level of iframe. Nested iframes are not told that they are overlapped.
<rdar://problem/8621567>
Created attachment 72740 [details] Patch
Comment on attachment 72740 [details] Patch r=mitz in person with comments.
http://trac.webkit.org/changeset/71194