AJAX ModalPopupExtender

Modular PopUp Extender is Appering in corner and then Displaying in center,

Here is my Code

<cc1:ModalPopupExtender ID="messageExtenderPopUp" runat="server" BehaviorID="programmaticModalPopupBehavior1"
            TargetControlID="btnPopUp" OkControlID="ButtonOk1" CancelControlID="ButtonCancel1"
            PopupControlID="messagePopup1" DropShadow="false" RepositionMode="RepositionOnWindowScroll"
            BackgroundCssClass="ui-widget-overlay">
        </cc1:ModalPopupExtender>
        <asp:Panel runat="server"  ID="messagePopup1" ClientIDMode="Static" Style="display: none"
            CssClass=" ui-dialog ui-widget ui-widget-content ui-corner-all  ui-draggable ui-resizable" >
            <div class="ui-dialog-titlebar ui-widget-header ui-corner-all ui-helper-clearfix">
                <span class="ui-dialog-title" id="Span1" runat="server">Information</span><a
                    id="CloseMsg" href="#" class="ui-dialog-titlebar-close ui-corner-all" role="button"><span
                        class="ui-icon ui-icon-closethick">close</span></a></div>
            <br />
            <table >
                <tr>
                    <td align ="left">
                        <div style="text-align: center; border: 0px;">
                            <asp:Label ID="lblConfirmMsg" runat="server"></asp:Label></div>
                    </td>
                </tr>
                <tr>
                    <td align="center">
                        <asp:Button runat="server" ID="btnConfirm" ClientIDMode="Static" Text="Ok" CssClass="inputBtnBig" />
                        <asp:Button runat="server" ID="btnClose" ClientIDMode="Static" Text="Close" CssClass="inputBtnBig" />
                    </td>
                </tr>
            </table>
            <br />
            <asp:Button runat="server" ID="ButtonCancel1" Style="display: none;" />
            <asp:Button runat="server" ID="ButtonOk1" Style="display: none;" Text="Yes" />
        </asp:Panel>