derive region from element data

          $parents[$name] = $parent ? array_merge($parents[$parent], [$parent]) : [];

          unset($list[$name]);

          // Determine the region for the row.

          $region_name = call_user_func_array($row['#region_callback'], [&$row]);

          $region_name = $row['region']['#value'] ?? '';

          if (!isset($regions[$region_name])) {

            $region_name = isset($regions['hidden']) ? 'hidden' : array_key_first($regions);

          }

          // Add the element in the tree.

          $target = &$trees[$region_name][''];