While laying out the boxes in a block formatting context, when we happen to need to get to the containing block, it should always be a descendant of the root container (or the root container itself). Nothing outside of the formatting context should be able to impact the boxes inside.
Created attachment 335923 [details] Patch
Committed r229658: <https://trac.webkit.org/changeset/229658>
<rdar://problem/38531372>