ASP.NET: ModalPopupExtender prevents button click event from firing

Here is what I'm trying to do: Click a button on my page, which in turn makes (2) things happen:

1. Display a ModalPopup to prevent the user from pressing any buttons or changing values
2. Call my code behind method, hiding the ModalPopup when finished

Here is the ASP markup:

<asp:UpdatePanel ID="UpdatePanel2" runat="server" ChildrenAsTriggers="true"
    UpdateMode="Always">
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="btnSaveData" EventName="Click" />
    </Triggers>
    <ContentTemplate>
        <asp:Panel ID="pnlHidden" runat="server" style="display: none;">
            <div>
            <h1>Saving...</h1>
            </div>
        </asp:Panel>
        <cc1:ModalPopupExtender ID="modalPopup"
            BackgroundCssClass="modalBackground" runat="server"
            TargetControlID="btnSaveData" PopupControlID="pnlHidden"
            BehaviorID="ShowModal">
        </cc1:ModalPopupExtender>
        <asp:Button ID="btnSaveData" runat="server" Text="Save Data"
            OnClick="btnSaveData_Click" />
    </ContentTemplate>
</asp:UpdatePanel>

Now, here is my code behind C# code:

protected void btnSaveData_Click(object sender, EventArgs e)
{
   UpdateUserData(GetLoggedInUser());
   modalPopup.Enabled = false;
}

Why doesn't this work? The ModalPopup displays perfectly, but the btnSaveData_Click event NEVER fires.

UPDATE: The first suggestion did not work for me. I also tried your second suggestion (insofar as it applied to me). One minor difference on my end is that there is no button on my modal panel (pnlHidden) -- it's just a message. I did try using Javascript events on the client side, which at least did fire concurrently with my server-side event. This was good news, but I can't seem to find or grab a handle on the ModalPo