Steps to reproduce the problem:
1. open index.html (attach)
2. scroll iframe
blue-block always on top
top part of iframe do not redrawing
Created attachment 61787 [details]
Created attachment 61788 [details]
problem in background color or image of page in iframe
Confirmed with r63561 on Leopard. Also happens in Safari/WebKit 4.0.5.
Still happens with Safari 5.1.4 and with current ToT.
Would be good to know when this regressed.
This was broken in mid-2010 already, so finding a regression range would be tricky.