PopupControlExtender To Show On MouseOver

This is an example of how to do it. Set unique BehaviorID to PopupControlExtender and call showPopup/hidePopup with it:

<ajaxToolkit:PopupControlExtender ID="PopupControlExtender1" runat="server" BehaviorID="pce" ............... />

<asp:Panel onMouseOver="$find('pce').showPopup();" onMouseOut="$find('pce').hidePopup();" />

NOTE: If you use it in a user control that could be used more than once on a page - give unique BehaviorID to each one.

Don't use <xhtmlconformance mode="Legacy"/> with ASP.NET AJAX

I do not have time to research why ASP.NET AJAX Toolkit is not working with <xhtmlconformance mode="Legacy"/> in web.config file, but anyway, hope I could help someone having my problem...

I had a calendar extender, but I got the error (as a javascript error):
this._pagerequestManager is null or not an object

The problem was that I used YAF component (YetAnotherForum.net) which uses this mode:
xhtmlconformance mode="Legacy"

PS: Hah, what I just understood is that the difference between mode="Legacy" and mode="Transactional" (default) is the forming of UniqueID of the controls (UniqueID property includes the identifier for the server control's naming container... more). For example, if you do not name a control, its ClientID is ctl0 or ctl00 depending on the mode mentioned above. If that control contains child controls, they would have the prefix ctl0 ot ctl00 as well.

See xhtmlConformance Element in MSDN.
See more about this.