How do I add a variable for menu.html.twig?

Question

I want a variable in menu.html.twig file. I tried adding the following hook in the theme's .theme file, but the template file doesn't print anything.

function my_theme_name_preprocess_menu(&$variables) {
  $account = \Drupal::currentUser();
  $variables['current_username'] = $account->getDisplayName();
}

The menu.html.twig template file contains the following markup.

<div class="title-container">
   <span class="item-title">{{ current_username }}</span>
</div>

Answer 1

Two issues. For preprocessing in general, a variable depending on the current user needs also cache metadata:

$variables['current_username'] = [
  '#markup' => \Drupal::currentUser()->getDisplayName(),
  '#cache' => [
    'contexts' => [
      'user',
    ],
    'tags' => [
      'user:' . \Drupal::currentUser()->id(),
    ],
  ],
];

Specific to menu templates. They are not normal template files, they contain a macro which is run for each menu item, where only the variables are available which are passed in the argument list:

{% macro menu_links(items, attributes, menu_level) %}

So you need to add the variable in the items menu tree so that it is available for the menu item you want to target.