It is sufficient to invalidate siblings before the mutation point.
<rdar://problem/38134480>
Created attachment 334992 [details] patch
Comment on attachment 334992 [details] patch Clearing flags on attachment: 334992 Committed r229288: <https://trac.webkit.org/changeset/229288>
I searched on Webkit GitHub with "183325" and see that it didn't back out. I am going to mark this as "RESOLVED FIXED". Thanks!