This is a regression that was born recently.
It is easy to reproduce using ManualTests/fixed-position.html
Created attachment 192180 [details]
Created attachment 192188 [details]
noamr's suggestion in IRC makes this patch more succinct.
Comment on attachment 192188 [details]
Clearing flags on attachment: 192188
Committed r145207: <http://trac.webkit.org/changeset/145207>
All reviewed patches have been landed. Closing bug.