Thank you Marijn for your fast response! Works like a charm!
And a very smooth backward compatibility for SS 2.3 as well.

I just adjusted your code a little bit for the 2.4.4 version. Now it is possible trigger all childknodes.

Hope it works for you.

<?php
/**
* TreeDropdown-like field that gives you a tree of items including an empty field, using ajax.
* Author: Marijn Kampf www.exadium.com
* Date:             24 Nov 2009
* Version:          2.0
* Revision date:      17 June 2010
* Changes:          Updated to work with SilverStripe 2.4, tree function added.
*/
class OptionalTreeDropdownField extends TreeDropdownField {
   private static $postTree = '</li></ul>';

   /**
    * Helper function to return the header (rather than defining same line twice).
    */
   function preTree() {
   	  
      return '<li id="0" class="l"> <a>' . _t('OptionalTreeDropdownField.NONE', "(None)", PR_MEDIUM, 'Non selected value of a dropdown') . '</a>';
   }

   /**
    * Return the site tree
    * For version 2.3 and earlier
    */
   /*function gettree() {
      echo $this->preTree();
      parent::gettree();
      echo $this->postTree;
   }*/

   /**
    * Get the whole tree of a part of the tree via an AJAX request with empty / none item prepended.
    *
    * @param SS_HTTPRequest $request
    * @return string
    * for version 2.4 and later
    */
   function tree($request) {
      return $this->preTree() . parent::tree($request) . $this->postTree;
   }
}
?>

Hi guys.

That was exactly what I was looking for! Thank you so much!
But there is a little bug in SS 2.4.5.
Everything is working as expected, as long as you select pages within the root level. If you open a subtree and try to select pages within it the pages are not selectable (Even the yellow hover highlighting is not working).

Any suggestions?

hi,

use DOM's SimpleTreeDropdownField:

	$f = new SimpleTreeDropdownField("PageLinkToID","Page link to", "SiteTree");
	$f->setHasEmptyDefault(true);

regards

pali

Thanks Pali - Very nice simple solution! :)