JIT fails the node info callback when item ID is an integer (not string) for all engines

Sorry – I was going to add some more comments, but got distracted. :-)

Below is the code I tested this with. It outputs the $id_string when any non-root item is clicked, but nothing is outputted when the root item is clicked. So it seems the function isn't called at all.

/**
 * Function providing JSON formatted information about a topic in a tree.
 *
 * @param $id_string
 *   A string of IDs in the tree leading up to the clicked item, on the form
 *   ID1__ID2__ID3 (where ID1 is the starting point for the tree path and ID3 is
 *   the clicked item). This is the way it's provided from The JIT.
 */
function sc_topictrees_info_callback($id_string) {
  // DEBUG START
  $item_info = array('title' => 'title', 'body' => $id_string);
  print drupal_json_encode($item_info);
  die;
  // DEBUG END

  // Get the ID we're really looking for.
  $id_array = explode('__', $id_string);
  $clicked_id = array_pop($id_array);

  // If the root object was clicked, just abort all output in a brutal way.
  if ($clicked_id == '_root') {
    // Brutal way:
    die;
  }

  // Get a custom-built view, created for providing tree item info.
  $view = views_get_view('sc_topictree_item_info');
  $view->set_arguments(array($clicked_id));
  $view->execute();

  // The JIT expects an array with the keys 'title' and 'body'. Let's make it.
  $item_info = array(
    // Note that the view is created to have a result called 'title' and
    // 'sc_description', but if you change the view you might have to change
    // the render_field.
    'title' => $view->render_field('title', 0),
    'body' => $view->render_field('sc_topic_description', 0),
  );

  // Return the info as JSON, then stop any further execution of Drupal.
  print drupal_json_encode($item_info);
  die;
}

And here's the menu hook, in case it could be a source of problem.

/**
 * Implements hook_menu().
 */
function sc_topictrees_menu() {
  $items['sc_topictrees/tree-info-callback/%'] = array(
    'page callback' => 'sc_topictrees_info_callback',
    'page arguments' => array(2),
    'title' => 'Information about tree items',
    'type' => MENU_CALLBACK,
    'access arguments' => array('access content'),
  );
  return $items;
}

I've been staring at various output and JS, trying to understand something.

My best guess right now is that the info callback fails when the ID is an integer rather than a string. When using node ID as identifier in JIT, the root item (and only the root item) gets an integer ID. All the others have IDs like "1__2".

When the root item is NOT a Drupal node, the ID becomes "_root", and things work.

Maybe it is some JS function that assumes that the ID is a string, and the callback is aborted when an error is thrown. Could be.
I don't know a line of JS, but found this that looks like a candidate: id = node['id'].replace(that.thisid + '_',''); (used at some place for fetching ID for a tree node)

In the demo, linked above, the root item has ID "jitnode_1", so that fits the pattern as well.

Ok, I got some more support for the hypothesis in #5. I tried changing the to/from IDs from node ID to node titles, and now the info box renders just fine.

Something goes wrong when the tree item ID is an integer.

Updating the title and the issue summary.

Awesome!
This patch solves the problem.