How to make a theme hook suggestion for menu according to region? I'm placing the same menu block in two different regions whereas different html markup for the list is needed. So analog to my previous question "How to make a theme hook suggestion for blocks according to region?" I need to know, how to get the current region inside the function MYMODULE_theme_suggestions_menu_alter, to add my own suggestion.
Joery Lemmens posted an answer to a similar question on Drupal's forums:

use Drupal\block\Entity\Block;

/**
 * Implements hook_preprocess_HOOK().
 *
 * Pass block region value to content so this can be used in
 * MYTHEME_theme_suggestions_menu_alter.
 */
function MYTHEME_preprocess_block(&amp;$variables) {
  if (isset($variables['elements']['#id'])) {
    $region = Block::load($variables['elements']['#id'])-&gt;getRegion();
    $variables['content']['#attributes']['region'] = $region;
  }
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 *
 * Provide region based menu suggestions.
 */
function MYTHEME_theme_suggestions_menu_alter(&amp;$suggestions, array $variables) {
  if (isset($variables['attributes']['region'])) {
    $suggestions[] = 'menu__' . $variables['menu_name'] . '__' . $variables['attributes']['region'];
  }
}


All credit to Joery for this elegant solution.