title: RegisterStartupScript doesn't appear to be working on page postback within update panel
body: <p>OK - so am working on a system that uses a custom datepicker control (I know there are other ones out there.. but for consistency would like to understand why my current issue is happening and fix it).</p>
  
  <p>So its a custom user control with a textbox and on Page_PreRender does this:</p>
  
  <pre><code>protected void Page_PreRender(object sender, EventArgs e)
      {
  
          string clientScript = @"
          $(function(){
          $('#" + this.Date1.ClientID + @"').datepicker({dateFormat: 'dd/mm/yy', constrainInput: true});
          });";
          Page.ClientScript.RegisterStartupScript(this.GetType(), this.ClientID, clientScript, true);
  
          //Type t = this.GetType();
          //if (!Page.ClientScript.IsStartupScriptRegistered(t, this.ClientID))
          //{
          //    Page.ClientScript.RegisterStartupScript(t, this.ClientID, clientScript, true);
          //}
      }
  </code></pre>
  
  <p>Ignore commented out stuff - that was me trying something different - didn't help.</p>
  
  <p>My issue is that this all works fine when I load the page.  But if I select something from a dropdownlist causing a page postback - when I click into my date fields they stop working.  As in I should be able to click into the textbox and a nice calendar control appears.  But after postback there is no nice calendar control appearing!</p>
  
  <p>It's currently all wrapped (in the hosting page) inside an update panel.  So I comment out the update panel stuff and the dates are working after page postback.  So it appears to be something related to that update panel.</p>
  
  <p>Any suggestions please?</p>
  
  <p>Thanks!!</p>