title: How to have a link in one placeholder open a ModalPopup in a different placeholder?
body: <p>I have a page with different placeholders. In one of them, I have a link that I want to open a modal popup in a second placeholder (using the ajaxtoolkit <code>ModalPopupExtender</code>) :</p>
  
  <pre><code>&lt;asp:Content ID="content1" ContentPlaceHolderID="placeholder1" Runat="Server"&gt;
     &lt;asp:LinkButton ID="link" runat="server" Text="Popup link" /&gt;
  &lt;/asp:Content&gt;
  
  &lt;asp:Content ID="content2" ContentPlaceHolderID="placeholder2" Runat="Server"&gt;
     &lt;asp:Panel ID="panel" runat="server" Text="Popup content" /&gt;
     &lt;ajaxToolkit:ModalPopupExtender ID="popup" runat="sever"
        TargetControlID="link"
        PopupControlID="panel"
        /&gt;
  &lt;/asp:Content&gt;
  </code></pre>
  
  <p>When doing as above, it fires me an exception, saying that <code>popup</code> cannot find <code>link</code> (which I understand, because they are in two different placeholders).</p>
  
  <p>How can I make this work ? I can think of something find <code>FindControl</code> in the code behind, but I don't really like to use this function, as it is pretty computationally expensive (especially with my nested layout).</p>