While computing the static position and traversing the ancestor chain, we can surely hit a positioned container (since we need to go all the way up to the initial containing block)
Created attachment 359001 [details]
Comment on attachment 359001 [details]
Clearing flags on attachment: 359001
Committed r239907: <https://trac.webkit.org/changeset/239907>
All reviewed patches have been landed. Closing bug.