There might be multiple layout roots per layout frame.
Created attachment 339275 [details] Patch
Created attachment 339431 [details] Patch
Comment on attachment 339431 [details] Patch Clearing flags on attachment: 339431 Committed r231318: <https://trac.webkit.org/changeset/231318>
All reviewed patches have been landed. Closing bug.
<rdar://problem/39948595>