title: Checkbox does not get checked/unchecked when used as control for CollapsiblePanelExtender
body: <p>Has anyone else tried to use checkbox as control to collapse/expand the AJAX CollapsiblePanelExtender?</p>
  
  <p>The panel collapses/expands fine when I am clicking the checkbox. But the checkbox itself won't get checked.</p>
  
  <p>Did that happen to you too?</p>
  
  <p>I know there's a work around this, but I can't rest until I understand why.</p>
  
  <p>Here's the code just in case someone wants to see:</p>
  
  <pre><code>&lt;asp:ScriptManager ID="ScriptManager1" runat="server"&gt;&lt;/asp:ScriptManager&gt;    
  &lt;div class="standard"&gt; 
      &lt;asp:UpdatePanel ID="UpdatePanelBespokeRates" runat="server"&gt;
          &lt;ContentTemplate&gt;       
              &lt;asp:CheckBox ID="checkbespoke" runat="server" AutoPostBack="False" Checked="false" Text="Click and unclick this checkbox" /&gt;
              &lt;/p&gt;
              &lt;asp:UpdatePanel ID="UpdatePanelBespoke" runat="server"&gt;
                  &lt;ContentTemplate&gt;
  
                      &lt;asp:CollapsiblePanelExtender ID="CollapsibleExtender2" runat="server" 
                          TargetControlID="PnlBespokeRates" CollapseControlID="checkbespoke" 
                          CollapsedSize="1" ExpandControlID="checkbespoke" SuppressPostBack="True" 
                          Enabled="True" Collapsed="True"&gt;&lt;/asp:CollapsiblePanelExtender&gt;
  
                      &lt;asp:Panel ID="PnlBespokeRates" runat="server" Visible="True" Height="300px" Width="200px" BackColor="White"&gt;
                           &lt;p&gt;Another Hello World text&lt;/p&gt;   
                      &lt;/asp:Panel&gt; 
  
                  &lt;/ContentTemplate&gt;
                  &lt;Triggers&gt;
                      &lt;asp:AsyncPostBackTrigger ControlID="checkbespoke" EventName="CheckedChanged" /&gt;
                  &lt;/Triggers&gt;
              &lt;/asp:UpdatePanel&gt;
  
          &lt;/ContentTemplate&gt;
      &lt;/asp:UpdatePanel&gt;
      &lt;p&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>in the Code behind it is:</p>
  
  <pre><code>protected void Page_Load(object sender, EventArgs e)
      {
          CollapsibleExtender2.ClientState = "true";
          CollapsibleExtender2.Collapsed = true;
      }
  </code></pre>