I        This condition is triggered when a client disconnects from the web server.


This isn't necessarily once a Webserver's response to an On XXX Request has been fully received, as HTTP allows a Connection: Keep-Alive request header to request multiple URIs over the same connection. In that scenario you will only receive a close once all of the pages are done and the timeout finishes, or the user navigates away and the web browser closes it early.

Read more on the request headers Connection, and Keep-Alive here.