Since the source field on event object is transient, it is very unlikely that
someone can deduce anything useful from opposite field. Deserialized window
could have never actually be activated or receive focus. So null seems to be
the only possible value for WindowEvent.opposite field after deserialization.

There was a similar issue with FocusEvent.opposite reported by a customer from
HP (see bug id 4715486). This was resolved by making the field explicitly
serialize and deserialize to null. Michael Warres pointed out that since the
observable behaviour in this case is the same as if the field was transient, we
can as well make it transient. So I believe the issue of 4715486 needs to be
revisited and both WindowEvent.opposite and FocusEvent.opposite must be
transient.