I have the code below to implement a dropdownlist with checkboxes. My problem is that every time i click a checkbox the dropdownlist closes and i need to reopen it to select more checkboxes. How do i make it so the dropdownlist dosn't close until i click off of it?

    <asp:Panel ID="pnl_Items" runat="server" BorderColor="Aqua" BorderWidth="1">
        <asp:CheckBoxList ID="cbl_Items" runat="server">
            <asp:ListItem Text="Item 1" />
            <asp:ListItem Text="Item 2" />
            <asp:ListItem Text="Item 3" />          
        </asp:CheckBoxList>
    </asp:Panel>

   <br />
    <asp:TextBox ID="tb_Items" runat="server"></asp:TextBox>
    <ajax:DropDownExtender  ID="TextBox1_DropDownExtender" 
                            runat="server" 
                            DynamicServicePath="" 
                            Enabled="True" 
                            DropDownControlID="pnl_Items" on 
                            TargetControlID="tb_Items">
    </ajax:DropDownExtender>