title: asp:DropDownList submits page on selection enter
body: <p>I have an asp.net page which has a panel with default button and update panel and dropdownlist inside the update panel. While I fill the form when I select an item with up/down keys and press enter to select it the form is being submitted. How can I fix this?</p>
  
  <p><strong>EDIT</strong>
  Some code:</p>
  
  <pre><code>&lt;asp:SqlDataSource ID="sdsCountries" runat="server" ConnectionString="cs"
  SelectCommand="SELECT CountryID, Title FROM Countries"/&gt;
  
  &lt;asp:DropDownList ID="ddlCountries" runat="server" DataSourceID="sdsCountries"
  DataValueField="CountryID" DataTextField="Title" AutoPostBack="True"
  OnSelectedIndexChanged="ddlCountries_SelectedIndexChanged"/&gt;
  
  &lt;asp:UpdatePanel ID="upState" runat="server" UpdateMode="Conditional"&gt;
      &lt;ContentTemplate&gt;
          &lt;asp:SqlDataSource ID="sdsStates" runat="server" ConnectionString="cs"
      SelectCommand="SELECT StateID, Title FROM States WHERE CountryID=@CountryID"&gt;
              &lt;SelectParameters&gt;&lt;asp:ControlParameter Name="CountryID"
           ControlID="ddlCountries" PropertyName="SelectedValue" DefaultValue="1"/&gt;
              &lt;/SelectParameters&gt;
          &lt;/asp:SqlDataSource&gt;
          &lt;asp:DropDownList ID="ddlStates" runat="server" DataSourceID="sdsStates"
          DataValueField="StateID" DataTextField="Title"/&gt;
      &lt;/ContentTemplate&gt;
      &lt;Triggers&gt;
          &lt;asp:AsyncPostBackTrigger ControlID="ddlCountries"
          EventName="SelectedIndexChanged" /&gt;
      &lt;/Triggers&gt;
  &lt;/asp:UpdatePanel&gt;
  </code></pre>