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 ModalPopupExtender or its BehaviorID. This doesn't work:

function showOverlay() {
    var popup = $find('modalPopup');
    popup.show();
}

popup is ALWAYS equal to null.

FINAL UPDATE: This is my final solution for getting this to work (Take specific note of the use of OnClientClick AND OnClick):

<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="hdnField" PopupControlID="pnlHidden"
        BehaviorID="ShowModal">
    <asp:HiddenField ID="hdnField" runat="server" />
    </cc1:ModalPopupExtender>
    <asp:Button ID="btnSaveData" runat="server" Text="Save Data"
        OnClientClick="showModal();" OnClick="btnSaveData_Click" />
    </ContentTemplate>
</asp:UpdatePanel>

Using this Javascript function:

function showModal() { $find('ShowModal').show(); }

... And this code behind:

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

Try this.

Create a dummy hidden field:

<asp:HiddenField ID="hdnField" runat="server" />

Set the TargetcontrolID = "hdnField" in your Modal Popup declaration.

In your btnSaveData_Click event, do this:

modalPopup.Show();

Try this. It is 100% working

 <asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:Button ID="Btnshow" runat="server" Text="Show" OnClick="Btnshow_Click" />
        <asp:Button ID="BtnTarget" runat="server" Text="Target" Style="display: none" />
        <asp:TextBox ID="TextBox1" runat="server">
        </asp:TextBox>
        <input type="button" value="Get" onclick="abc()" />
        <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="BtnTarget"
            PopupControlID="Panel1">
        </asp:ModalPopupExtender>
        <asp:Panel ID="Panel1" runat="server" BackColor="Black" Width="300px" Height="300px">
            <asp:UpdatePanel ID="UpdatePanel2" runat="server" ChildrenAsTriggers="false" UpdateMode="Conditional">
                <ContentTemplate>
                    <asp:Button ID="BtnHide" runat="server" Text="Hide Button" OnClick="BtnHide_Click" />
                </ContentTemplate>
                <Triggers>
                    <asp:AsyncPostBackTrigger ControlID="BtnHide" EventName="Click" />
                </Triggers>
            </asp:UpdatePanel>
        </asp:Panel>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="Btnshow" EventName="Click" />
    </Triggers>
</asp:UpdatePanel>

Server side code

protected void Btnshow_Click(object sender, EventArgs e)
{
    ModalPopupExtender1.Show();
}
protected void BtnHide_Click(object sender, EventArgs e)
{
    ModalPopupExtender1.Hide();
}

First attempt: Try to set your ButtonID into OkControlID Tag and try again

OR

Second attempt: Call your event over javascript there seems to be some problems with click events

<div> 
    <cc1:ModalPopupExtender PopupControlID="Panel1"  
         ID="ModalPopupExtender1" 
         runat="server" TargetControlID="LinkButton1" OkControlID="Ok"  
         OnOkScript="__doPostBack('Ok','')"> 
    </cc1:ModalPopupExtender> 
    <asp:LinkButton ID="LinkButton1" runat="server">LinkButton</asp:LinkButton>  
</div>         

<asp:Panel ID="Panel1" runat="server"> 
    <asp:Button ID="Ok" runat="server" Text="Ok" onclick="Ok_Click" />             
</asp:Panel>

From this example you can easily control panel show up depends on conditions instead of just immediately show up panel once you click button.

<asp:Button ID="btnAdd" runat="server" OnClick="btnAdd_Click"/>
<asp:HiddenField ID="hdnField" runat="server" />
<ajaxToolkit:ModalPopupExtender runat="server" 
  TargetControlID="hdnField" 
  ID="btnAdd_ModalPopupExtender"
  PopupControlID="pnlPrintName">
</ajaxToolkit:ModalPopupExtender>
<asp:Panel ID="pnlPrintName" runat="server">
.
.
</asp:Panel>

//Server side code:
protected void btnAdd_Click(object sender, EventArgs e)
{
  if( dt.Rows.Count == 0 )
  {
     btnAdd_ModalPopupExtender.Show();
  }
  else
  {
     add();
  }
}

In code behind, you can do this:

if (true)
{
    var script = @"Sys.Application.add_load(function() { $find('behavoirIDModal').show(); });";
    ScriptManager.RegisterStartupScript(this, GetType(), "Show", script, true);
}

Change this 'behavoirIDModal'