Hello,
I have an ascx control which contains both a CollapsiblePanelExtender and ModalPopupExtender controls. The CollapsiblePanelExtender.Collapsed=True in the xml markup. It appears that when I call the Show() method of ModalPopupExtender from C# code behind the pop up does not show up intill I click on expand panel button. When I click on expand panel the popup shows up and works as intended. It seems that the Collapsed property hides the popup. The fix is not to set the CollapsiblePanelExtender.Collapsed=True in the xml markup and then it works as expected. I realize that it's not a huge issue but just wanted to give a heads up to the community.
Thanks, Yuriy