It is possible to have an object that toggles between being fixed and being static that will still scroll as if it is fixed when it has become static. The attached test case demonstrates the bugs.
Created attachment 184591 [details] Test case
Created attachment 184593 [details] Patch Unfortunately, I was unable to create a test for this because scrollingStateTreeAsText() will make that commit that wasn't happening happen anyway.
Thanks, Simon! http://trac.webkit.org/changeset/140747