title: Asp.net updatepanel Listbox not refreshing layout
body: <p>so my problem is fairly simple i have 2 list-boxes and one button.
  when an item is selected in listbox2 and the button is clicked i would like to add the selected item to listbox1. </p>
  
  <p>And it all seams to work until I add the second item. then listbox1  doesn't refresh it's items... </p>
  
  <pre><code>&lt;body&gt;
  &lt;form id="form1" runat="server"&gt;
      &lt;asp:ScriptManager runat="server"&gt;&lt;/asp:ScriptManager&gt;
      &lt;div&gt;
          &lt;asp:UpdatePanel ID="UpdatePanel1" runat="server"&gt;
              &lt;ContentTemplate&gt;
                  &lt;asp:ListBox ID="ListBox1" runat="server"&gt;&lt;/asp:ListBox&gt;
              &lt;asp:ListBox ID="ListBox2" runat="server"&gt;&lt;/asp:ListBox&gt;
                  &lt;asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" /&gt;
              &lt;/ContentTemplate&gt;
          &lt;/asp:UpdatePanel&gt;
      &lt;/div&gt;
      &lt;/form&gt;
  &lt;/body&gt;
  public partial class Default2 : System.Web.UI.Page
  {
      protected void Page_Load(object sender, EventArgs e)
      {
          if (!IsPostBack)
          {
              ListBox2.Items.Add(new ListItem("1", "1"));
              ListBox2.Items.Add(new ListItem("2", "2"));
              ListBox2.Items.Add(new ListItem("3", "3"));
              ListBox2.Items.Add(new ListItem("4", "4"));
              ListBox2.Items.Add(new ListItem("5", "5"));
          }
      }
      protected void Button1_Click(object sender, EventArgs e)
      {
          if (ListBox2.SelectedIndex != -1)
          {
              ListBox1.Items.Add(ListBox2.SelectedItem);
          }
      }
  }
  </code></pre>