When you have a object with a relative position, inside an object with overflow hidden, inside a fixed object, the relatively positioned object gets clipped when you scroll the page. The sample page has a simple set of examples: http://eeoh.com.au/bugTesting/safari_fixed_overflow_relative_scroll_bug.html
Created attachment 9099 [details] Simple test case example in html
Confirmed with WebKit r15133 and WebKit 418.8. Thanks for the test case.
I have found an addition to this bug, as well as the original case the same things happens with an overflow hidden object, inside an overflow hidden object, inside a fixed object. I'll attach a more complete example.
Created attachment 11579 [details] Revised html test case
Created attachment 11580 [details] revised test case
The bug is broader than I had thought. In addition to the original bug... If you have a positioned object, or an overflow hidden object inside a fixed object with overflow hidden then all the positioned objects get clipped on scroll.
Created attachment 11581 [details] revised test case This revised test case includes the revised bug and some clarifying examples (hopefully). If I can simplify matters further I will...
*** Bug 8788 has been marked as a duplicate of this bug. ***
*** Bug 11937 has been marked as a duplicate of this bug. ***
Created attachment 12130 [details] Add "fixed" flag to cached clip rects No layout test regressions.
Comment on attachment 12130 [details] Add "fixed" flag to cached clip rects r=me
Committed revision 18491.