EDIT

the problem seems mainly to be caused by the NavigationManagerBean not releasing the references to RegistrationWorksetViewBeans contained in currentView and by the PopupViewRegistration keeping the RegistrationWorksetViewBean in the popupViewFieldMap where its used as key.

Resources which are still in memory after closing a browser window should be destroyed automatically once the next heardbeat has reached the server.
It is defined by the @VaadinServletConfiguration.InitParameterName(value="heartbeatInterval"):

5.8.5. UI Expiration
UI instances are cleaned up if no communication is received from them after some time. If no
other server requests are made, the client-side sends keep-alive heartbeat requests. A UI is
kept alive for as long as requests or heartbeats are received from it. It expires if three consecutive
heartbeats are missed.

The heartbeatInterval is at 5 minutes (300 seconds) per default.