[[WebApp VB.net ]] ปัญหา ModalPopupExtender กระพริบตอน Load เพจครั้งเเรกเเก้อย่าไรครับ

<asp:scriptmanager id="ScriptManager1" runat="server">
</asp:scriptmanager>

<asp:button id="Button1" runat="server" text="Button" />

<cc1:modalpopupextender id="ModalPopupExtender1" runat="server" 
	cancelcontrolid="btnCancel" okcontrolid="btnOkay" 
	targetcontrolid="Button1" popupcontrolid="Panel1" 
	popupdraghandlecontrolid="PopupHeader" drag="true" 
	backgroundcssclass="ModalPopupBG">
</cc1:modalpopupextender>

<asp:panel id="Panel1" style="display: none" runat="server">
	<div class="HellowWorldPopup">
                <div class="PopupHeader" id="PopupHeader">Header</div>
                <div class="PopupBody">
                    <p>This is a simple modal dialog</p>
                </div>
                <div class="Controls">
                    <input id="btnOkay" type="button" value="Done" />
                    <input id="btnCancel" type="button" value="Cancel" />
		</div>
        </div>
</asp:panel>