RadComboBox with Checkboxes AJAX problem

I have created radcombobox with list of checkboxes. User can select multiple checkboxes and when he check some item label on page must be updated (this.label.text += someValue). I added Ajax:UpdatePanel with async trigger on that radcombobox but problem is when user check item dropdown list close it self :( How can I prevent closing dropdown list? Here what I have tried:

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<telerik:RadComboBox runat="server" ID="rcb" Width="200px" HighlightTemplatedItems="true" 
    AllowCusto开发者_开发技巧mText="true" Text="Select Item" MaxHeight="250px" EnableTextSelection="false" AutoPostBack="true"
    OnClientSelectedIndexChanging="OnClientSelectedIndexChanging()">
    <Items>
        <telerik:RadComboBoxItem Value="0" Text="Select..." />
        <telerik:RadComboBoxItem Value="1" Text="Small" />
        <telerik:RadComboBoxItem Value="2" Text="Medium" />
        <telerik:RadComboBoxItem Value="3" Text="Large" />
    </Items>
    <ItemTemplate>        
            <asp:CheckBox onclick="stopPropagation(event);" ID="chk_Category" runat="server" Text="test" AutoPostBack="true" OnCheckedChanged="CheckBox1_CheckedChanged" />       
    </ItemTemplate>    
</telerik:RadComboBox>

<dnn:label ID="lbl" runat="server" Text="nothing" />

</ContentTemplate>
<Triggers>
    <asp:AsyncPostBackTrigger ControlID="rcb"/>    
</Triggers>
</asp:UpdatePanel>

<script type="text/javascript" language="javascript">
        // <![CDATA[
    function stopPropagation(e) {        
        e.cancelBubble = true;
        if (e.stopPropagation) {
            e.stopPropagation();
        }
    }
    function OnClientSelectedIndexChanging(item) {       
        return false;
    }

                // ]]>
</script>

Typically, the drop down closes when you click on the item; we use a checkbox in the combo box template, and only experienced the closing when clicking on the item itself (which selects the item, and that becomes confusing).

The issue here is that each checkbox posts back to the server, so that is the most likely cause. Do you have to send the checkbox response back to the server after clicking it? An alternative approach is to read the checkbox controls from each item in bulk, or every time an item is checked, store the values of the checked items in a hidden control.

Alternatively, as an FYI, for the Q2 release of 2011, upcoming is this feature: Multiple select with checkboxes mode. So checkboxes will be a default feature of the combo box. If you have support, you can upgrade shortly.

HTH.