In short: yes, there is a happens-before relationship imposed between actions of the thread calling invokeLater
/invokeAndWait
and actions on the EDT of the runnable thereby submitted. Without that the sanity of the whole API would be at stake.
Unfortunately, it is hard to come by any authoritative source which would confirm that. That happens with a lot of stuff regarding Swing and concurrency.
For a bit more information, refer to this answer by trashgod, a long-time Swing guru.