A method comprising: receiving, from a web browser, a request for content to be inserted into a sub-document that is nested inside a main document; and transmitting to said web browser, in response to the request: said content, and a client-side script that, when inserted by said web browser into said sub-document: (i) listens to software methods that attempt to invoke a user event without an action by a user of said web browser, (ii) analyzes said user event to determine if said user event is configured to cause said web browser to navigate away from said main document.