In most of the cases the parent is not found, probably because the child is not a direct descendant of the parent returned by parentObjectUnignored(). We need to handle the case of find() returning notFound.
Created attachment 366601 [details]
Committed r243864: <https://trac.webkit.org/changeset/243864>
Returning -1 when the parent is not known seems consistent with the docs at https://developer.gnome.org/atk/stable/AtkObject.html, so another (late) lgtm