Hidden panel does not display correctly when displayed

I have a simple AlwaysVisibleControlExtender that extends a small bar containing a Loading ... message and an animated GIF. This is all inside the UpdateProgress control, so it only shows when my application is processing. I have it set to display in the top center. It works great, but I noticed that it shows slightly to the right of where it should be. If I resize the window slightly, the panel snaps into position.

My guess is that it calculates where the center is before the UpdateProgress control allows the Panel to be rendered, so the panel is zero-width. Only after displaying the panel does it calculate the center correctly. Is there a workaround for this?

Update: Here's the markup:

<%@ Page Language="VB" MasterPageFile="~/Compass.master" AutoEventWireup="false" CodeFile="SubmissionPrep.aspx.vb" Inherits="SubmissionPrep" Title="Untitled Page" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajax" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
    <asp:UpdatePanel ID="UpdatePanel" runat="server" UpdateMode="Always">
        <ContentTemplate>
            <%-- Data and Controls here --%>
            <asp:UpdateProgress ID="ProcessingUpdateProgress" runat="server">
                <ProgressTemplate>
                    <asp:Panel ID="ProcessingPanel" runat="server" CssClass="loading">
                        Processing...
                        <img src="images/ajax-loader.gif" alt="" />
                    </asp:Panel>
                    <ajax:AlwaysVisibleControlExtender ID="ProcessingAlwaysVisibleControlExtender" runat="server"
                        TargetControlID="ProcessingPanel" VerticalSide="Top" HorizontalSide="Center">
                    </ajax:AlwaysVisibleControlExtender>
                </ProgressTemplate>
            </asp:UpdateProgress>
        </ContentTemplate>
    </asp:UpdatePanel>
</asp:Content>