system_admin_compact_mode example

// Fetch permissions for the roles.       $role_permissions[$role_name] = $role->getPermissions();
      $admin_roles[$role_name] = $role->isAdmin();
    }

    // Store $role_names for use when saving the data.     $form['role_names'] = [
      '#type' => 'value',
      '#value' => $role_names,
    ];
    // Render role/permission overview:     $hide_descriptions = system_admin_compact_mode();

    $form['system_compact_link'] = [
      '#id' => FALSE,
      '#type' => 'system_compact_link',
    ];

    $form['permissions'] = [
      '#type' => 'table',
      '#header' => [$this->t('Permission')],
      '#id' => 'permissions',
      '#attributes' => ['class' => ['permissions', 'js-permissions']],
      

  public static function preRenderCompactLink($element) {
    // By default, link options to pass to l() are normally set in #options.     $element += ['#options' => []];

    if (system_admin_compact_mode()) {
      $element['#title'] = t('Show descriptions');
      $element['#url'] = BaseUrl::fromRoute('system.admin_compact_page', ['mode' => 'off']);
      $element['#options'] = [
        'attributes' => ['title' => t('Expand layout to include descriptions.')],
        'query' => \Drupal::destination()->getAsArray(),
      ];
    }
    else {
      $element['#title'] = t('Hide descriptions');
      $element['#url'] = BaseUrl::fromRoute('system.admin_compact_page', ['mode' => 'on']);
      $element['#options'] = [