Ticket #28801: class-wp-walker.php.patch

src/wp-includes/class-wp-walker.php

@@ -214 +214 @@
                  */
                 $top_level_elements = array();
                 $children_elements  = array();
+                
+                /*
+                 * Default $root to the first element
+                 * Used in the case that there are no top level elements
+                 */
+                $first = array_slice( $elements, 0, 1 );                        
+                $root = $first[0];
+                $element_ids = array_map(function($element){ return $element->ID; }, $elements);
+                
                 foreach ( $elements as $e) {
-                        if ( empty( $e->$parent_field ) )
+                        // If the parent_field is not blank OR in the list of elements update to be the root element
+                        if ( !empty( $e->parent_field) && !in_array($e->parent_field, $element_ids))
+                                $root = $e;
+                        elseif ( empty( $e->$parent_field ) )
                                 $top_level_elements[] = $e;
                         else
                                 $children_elements[ $e->$parent_field ][] = $e;
@@ -223 +235 @@
 
                 /*
                  * When none of the elements is top level.
-                 * Assume the first one must be root of the sub elements.
+                 * Set the element buckets based on the root defined above
                  */
                 if ( empty($top_level_elements) ) {
-
-                        $first = array_slice( $elements, 0, 1 );
-                        $root = $first[0];
-
                         $top_level_elements = array();
                         $children_elements  = array();
                         foreach ( $elements as $e) {