I have been ask if RAP can resume a session if a user has navigated away from a page or close/reopened their browser. I know that currently this is not supported, the GET request restarts the session and the Entry point is called again.
Since the widget state is stored in the session, it makes sense that in theory, the client state on the browser should be recoverable, providing code is written to do so.
The issue also comes with session replication. If the replication between servers is asynchronous and a failure happens after a response is sent to the client but before the changes to the session have been replicated the browser will be out of sync with session that is available to the fail-over server.