Restrict modalpopupextender position moving when iphone zoom

Question

Hi,
        I have added modalpopupextender in my project. When i open a project in iphone and zoom it means the modalpopupextender window position is changed again and again. The position is changed when i zooming, so kindly give solution for that. 

This is my .aspx code

<ajaxToolkit:ModalPopupExtender ID="popup_LoginAlert" BackgroundCssClass="modalBackground"

                    DropShadow="false" runat="server" TargetControlID="hdn_LoginAlert" PopupControlID="pnl_LoginAlert"

                    Enabled="true" PopupDragHandleControlID="pnl_LoginAlert" RepositionMode="RepositionOnWindowScroll" />