title: TextBox loses value after post back with TextMode Number
body: <p>Got weird problem. I have simple page with <code>TextBox</code>:</p>
  
  <pre><code>&lt;asp:ScriptManager runat="server" /&gt;
  &lt;asp:UpdatePanel runat="server"&gt;
      &lt;ContentTemplate&gt;
          &lt;asp:TextBox ID="amountTextBox" runat="server" TextMode="Number" /&gt;
          &lt;asp:Button runat="server" Text="Confirm" OnClick="confirmButton_Click" /&gt;
      &lt;/ContentTemplate&gt;
  &lt;/asp:UpdatePanel&gt;
  </code></pre>
  
  <p>After <code>Button</code> click I try to get <code>TextBox</code> value:</p>
  
  <pre><code>public partial class test : Page {
      protected void confirmButton_Click(object sender, EventArgs e) {
          var answer = amountTextBox.Text;
      }
  }
  </code></pre>
  
  <p>but it's empty. If I remove <code>UpdatePanel</code>, I manage to get the value. If I leave <code>UpdatePanel</code> and remove <code>TextMode</code> property, I manage to get the value. But why can't I have <code>TextBoxt</code> with <code>TextMode</code> set to <code>Number</code> in <code>UpdatePanel</code>?</p>