asp.net ajax popupcontrol items show briefly on page

I have a usercontrol on my page. The user control contains an ajax updatepanel with a listbox in it. There is also a PopupControlExtender that is tied to that panel so when the user clicks on something the popup shows. That is all working fine except when my page initially loads I briefly see all the listbox ite开发者_运维百科ms and then they go away.

Got it: style="display:none" on the updatepanel