I had a bit of a nightmare trying to get the modal popup extender working when viewing a webpage containing this Ajax control in IE6. My thoughts regarding IE6 are best left out of this!! After some digging I found several articles that pointed to a variety of things - this is what worked for me.
As I was working with a page which relied on a series of masters I could not change the “doctype”.
This works with v1.0.10123.0
1. Change the file Common.js which is found at AjaxControlToolkit\Common\Common.js
2. Search for a method called 'getClientBounds'
3. Replace switch with
4. Important part is the next bit - as absolute positioning is not correctly implemented in IE6. Look for the initalize function and look for the background i.e.
replace this line:
and this line:
This should solve the problem.