The websocket disconnection issue has been fixed in the server build WCS3 517.

Websocket connecton was not dropped by server when client connection is lost due the network condition.

Generally websocket client sends pings to server and receives pong-requestst for the pings. When you close browser, it sends disconnection websocket frame to indicate connection lost.
However if we remove network cable from PC, browser does not send such disconnect frame. That’s why we need a server-side ping/pong mechanizm to keep the connection alive.