Summary of Zoom chat:

1. Things that should hold at this point:
   • IsRelocated() should be true on owned children.
   • The owner (parent) should be in mARIAOwnsHash.
   • We cannot assume that ContentRemoved runs inside WillRefresh. Even if it does, we can't guarantee it will run after the relocation list is processed. So, we can't rely on relocations to handle this.
2. Probably need to do this somewhere in UncacheChildrenInSubtree.
3. If a child IsRelocated(), put it back where it belongs; don't remove it. See PutChildrenBack... but that has to be called on the parent and it deals with multiple children,so that might make things interesting.
   • aria-owns can be used to reorder children without moving them across subtrees. We do want to remove the child in that case. So we might need to check whether the child has the same parent after "putting it back" and remove it if it does.

This revision modifies UncacheChildrenInSubtree such that removed but relocated
accessibles that are aria-owned are actually relocated to their proper parent
after the removal of the formerly-aria-owning parent. On the way to
accomplishing that goal, this revision adds PutChildBack, a method called by
PutChildrenBack and now also called by UncacheChildrenInSubtree. We don't always
want to put all the children back - this function lets us manage it one at a time.
Finally, this revision adds tests which verify that the functionality works
as intended.

https://hg.mozilla.org/mozilla-central/rev/edbaacc4f3d6

https://hg.mozilla.org/mozilla-central/rev/dcb5003e4c19