The Ajax Control Toolkit has a PopupExtender module that is used throughout the library in whichever controls need to show above other controls. I wanted to use the Calendar Extender in my web site, but the calendar appeared underneath other controls. I checked it out and it had a zIndex of 1000, which should have been enough. I took me an hour to realise that in the toolkit code zIndex was a property of the div element, not of the div style!
A download of the latest version from Feb 29 shows the problem is still there. The fix? go to the PopupExtender folder in the source code, open the PopupBehaviour.js file, search for a line that looks like this:
element.zIndex = 1000;and replace it with
element.style.zIndex = 1000;. Now it works!
The issue is already in the AjaxControlToolKit issue tracker, but it was not addressed yet.