On one drupal site, I use a multilevel menu as the main menu and set it to 'expand all' in the menu block settings. I don't know, if this is the usual behaviour - but with this setup I loose the active trails indicator on the menu array in twig. It took some while to find a solution for this problem ... here are the functions:
/**
* Implements hook_preprocess_menu().
*/
function zzz_helpers_preprocess_menu(&$var, $hook) {
// Get the current path.
$currentRoute = \Drupal::routeMatch()->getRouteName();
$currentParams = \Drupal::routeMatch()->getRawParameters();
$params = [];
foreach ($currentParams as $k => $v) {
$params[$k] = $v;
}
$menu_link_manager = \Drupal::service('plugin.manager.menu.link');
$menu_link = $menu_link_manager->loadLinksByRoute($currentRoute, $params);
if (is_array($menu_link) && count($menu_link)) {
$uuid = key($menu_link);
$menu_link = reset($menu_link);
if ($menu_link->getParent()) {
$parents = $menu_link_manager->getParentIds($menu_link->getParent());
}
$parents[$uuid] = $uuid;
}
_zzz_active_trail_helper($var['items'], $parents);
}
function _zzz_active_trail_helper(&$menu, $parents) {
foreach ($menu as $key => $item) {
if (in_array($key, $parents)) $menu[$key]['attributes']['class'] = 'current';
if (!empty($item['below'])) {
$arr = $menu[$key]['below'];
_zzz_active_trail_helper($arr, $parents);
$menu[$key]['below'] = $arr;
}
}
}