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.
Created attachment 72740 [details]
Comment on attachment 72740 [details]
r=mitz in person with comments.