asp.net ajax popupcontrol项目在页面上简要显示

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 items and then they go away.I have a usercontrol on my page. The user contr