I believe this is a Jetty issue. I have not seen any browsers close WebSocket connections due to inactivity nor have I encountered other WebSocket servers that timeout WebSocket connections.
Jetty is (was) primarily focused on building HTTP based application servlets. In that context, HTTP connections need to be cleaned up pretty aggressively and HTTP was not designed for long-lived connections so having a short default timeout is reasonable.
I've not seen the precise problem you described (closing even with activity) but I do see WebSocket connections closed after 30 second of inactivity. It's possible that in older versions of Jetty or in the current version for some other reason, the timer is not reset by WebSocket activity. I get around this by using the setMaxIdleTime method on my BlockingChannelConnector object to set the timeout value to Integer MAX_VALUE.