I don't have a clue how to work around this problem and I'm open to any suggestions on the topic....
In Internet Explorer it appears that the onbeforeunload event is fireing despite the fact that it is not a full page postback to the server that is occurring.
This is causing a problem because I'm displaying a <div> that blocks the page's content during the onbeforeunload event (in order to be consistent with what happens when I block the controls and display a "processing message" during partial page, asynchronous requests).
The problem is that the control that is supposed to block the page during the full page request to the server (during the onbeforeunload event) is also being displayed when the partial page request to the server is being made (via an ASP.NET UpdatePanel).
When the Ajax Request returns, the control that was supposed to block the page during a full page unload remains showing...
I don't really want both of the controls to show at the same time and would like to prevent the one that is supposed to be only be displayed during full page requests from being displayed during asynchronous requests.
-Frinny