It is sufficient to invalidate siblings before the mutation point.
Created attachment 334992 [details]
Comment on attachment 334992 [details]
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!