Those vertical position values are not estimations (or at least they should not be)
<rdar://problem/59288794>
Created attachment 390190 [details] Patch
Created attachment 390193 [details] Patch
Comment on attachment 390193 [details] Patch Clearing flags on attachment: 390193 Committed r256100: <https://trac.webkit.org/changeset/256100>
All reviewed patches have been landed. Closing bug.