It is only set by ::forceLayout(). Just convert the subtree layout to full right before calling layout.
<rdar://problem/35111012>
Created attachment 324502 [details] Patch
Comment on attachment 324502 [details] Patch Clearing flags on attachment: 324502 Committed r223821: <https://trac.webkit.org/changeset/223821>
All reviewed patches have been landed. Closing bug.