Hi HankAnzis,

Thanks for your response.

According to your description, I can reproduce the scenes you discribed.

If you do not want the pop up recentering when page scroll, you should set the attribute "RepositionMode" as "none".

<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" BackgroundCssClass="modalBackground"
                CancelControlID="btnClose" TargetControlID="lbBtnPopUp" PopupControlID="pnlPopUp"
                DropShadow="true" Drag="true" PopupDragHandleControlID="pnlPopUp" RepositionMode="None" >
</asp:ModalPopupExtender>

Please try it again, if any doubt, please feel free to back. Thanks.

Regards!