To reproduce: 1. Go to http://webkit.org/ 2. Scroll up past the top of the page As you scroll past the top, a linen pattern will appear at the top of the WebView. But notice that the linen pattern moves down with the page, rather than staying in place and being revealed by the page. (If you instead scroll down past the bottom, the linen pattern will stay in place and be revealed by the page, as desired.)
<rdar://problem/9833542>
I can't reproduce this anymore. Maybe my eyes went crazy.