One idiosyncrasy of the ModalPopupExtender is that its HTML content is actually visible while the page is loading, until the $create() call initializes it. If you have a lengthy load time, as I did with a ListView full of records, the visibility is noticeable...
I was using the ComboBox control of Microsoft’s Ajax Control Toolkit and ran into two issues. When its initially hidden (its inside something with a style of display:none) and you make it visible, the toggle button and actual dropdown list are incorrectly...