ModalPopupExtender AJAX flashes when page loads

ModalPopupExtender AJAX, I don't quite understand it and it likes to do stuff I don't know where it comes from.

For instance, in a project having a pop up on the master page and switching between pages the darn pop up flashes when page loads.  

solution:  add the style="display: none"

 

more at this link another-modal-popup-extender-question