If you get the message, there's no communication with the server. As most every mouseclick results in something having to be done on the server (except opening and closing windows), there's no point in allowing mouseclicks when communication with the server is impossible (it would in fact be dangerous, as in the interval between the click being registered on the client and the server handling it, maybe minutes later in really bad cases, things could have changed to where that click is no longer needed or even wanted).
