WebSession.TimedOut
From Xojo Documentation
Event
Fires when the user's browser has been idle for the time interval specified by the WebSession.Timeout property.
Notes
WebSession.TimedOut will only be called if you set the WebSession.Timeout property and then only if the user doesn't enter text or move their mouse for that period. It does not tell you about the browser disconnecting.
See Also
WebApplication.Timeout, WebApplication.SessionTimeout, Timeout properties