Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/menu_ui/menu_ui.admin.js

@@ -0,0 +1,68 @@
+/**
+ * @file
+ * Menu UI admin behaviors.
+ */
+
+(function ($) {
+
+  "use strict";
+
+  /**
+   *
+   * @type {Drupal~behavior}
+   */
+  Drupal.behaviors.menuUiChangeParentItems = {
+    attach: function (context, settings) {
+      var $menu = $('#edit-menu').once('menu-parent');
+      if ($menu.length) {
+        // Update the list of available parent menu items to match the initial
+        // available menus.
+        Drupal.menuUiUpdateParentList();
+
+        // Update list of available parent menu items.
+        $menu.on('change', 'input', Drupal.menuUiUpdateParentList);
+      }
+    }
+  };
+
+  /**
+   * Function to set the options of the menu parent item dropdown.
+   */
+  Drupal.menuUiUpdateParentList = function () {
+    var $menu = $('#edit-menu');
+    var values = [];
+
+    $menu.find('input:checked').each(function () {
+      // Get the names of all checked menus.
+      values.push(Drupal.checkPlain($.trim($(this).val())));
+    });
+
+    $.ajax({
+      url: location.protocol + '//' + location.host + Drupal.url('admin/structure/menu/parents'),
+      type: 'POST',
+      data: {'menus[]': values},
+      dataType: 'json',
+      success: function (options) {
+        var $select = $('#edit-menu-parent');
+        // Save key of last selected element.
+        var selected = $select.val();
+        // Remove all existing options from dropdown.
+        $select.children().remove();
+        // Add new options to dropdown. Keep a count of options for testing later.
+        var totalOptions = 0;
+        for (var machineName in options) {
+          if (options.hasOwnProperty(machineName)) {
+            $select.append(
+              $('<option ' + (machineName === selected ? ' selected="selected"' : '') + '></option>').val(machineName).text(options[machineName])
+            );
+            totalOptions++;
+          }
+        }
+
+        // Hide the parent options if there are no options for it.
+        $select.closest('div').toggle(totalOptions > 0).attr('hidden', totalOptions === 0);
+      }
+    });
+  };
+
+})(jQuery);