Issue #3520680 by ressa, dydave: Improved Admin Toolbar settings form page by...

  public function buildForm(array $form, FormStateInterface $form_state) {

    $config = $this->config('admin_toolbar.settings');

    // Add maximum 'Menu depth' select field with a range of values: 1 to 9.

    $depth_values = range(1, 9);

    $form['menu_depth'] = [

      '#type' => 'select',

      '#title' => $this->t('Menu depth'),

      '#description' => $this->t('Maximum depth of displayed nested menu items.'),

      '#default_value' => $config->get('menu_depth'),

      '#options' => array_combine($depth_values, $depth_values),

    // Add an introduction text to module's settings form.

    $form['settings_form_help_intro'] = [

      '#type' => 'markup',

      '#markup' => $this->t('The Admin Toolbar module provides a better user experience for the default Drupal Toolbar.<br/>It is a drop-down menu that allows quicker access to all the administration pages in a more efficient way, with fewer clicks and less scrolling.<br/><br/>The following settings mostly provide advanced configuration of the JavaScript behavior of the Toolbar: sticky and hoverIntent.'),

    ];

    // Add 'sticky behavior' wrapper as a 'fieldset' so it stays displayed.

      '#prefix' => $this->t("By default, the Admin Toolbar sticky behavior is <em>enabled</em> so it stays at the top of the browser window when scrolling up or down.<br/>Select <em>Disabled</em> to disable Admin Toolbar's sticky behavior so it stays at the top of the page when scrolling up/down and does not follow the browser window."),

      '#options' => [

        'enabled' => $this->t('Enabled'),

        'hide_on_scroll_down' => $this->t('Disabled, show on scroll-up'),

        'disabled' => $this->t('Disabled'),

        'hide_on_scroll_down' => $this->t('Disabled: Hide on scroll-down, show on scroll-up'),

      ],

      '#default_value' => $config->get('sticky_behavior') ?: 'enabled',

    ];

      ],

    ];

    /* Add Advanced form settings. */

    // Add 'Advanced settings' wrapper as 'details' so it could be collapsed.

    $form['advanced_settings_wrapper'] = [

      '#type' => 'details',

      '#title' => $this->t('Advanced settings'),

      '#description' => $this->t('Advanced settings for the Admin Toolbar module.'),

      // Collapsed by default.

      '#open' => FALSE,

    ];

    // Add maximum 'Menu depth' select field with a range of values: 1 to 9.

    $depth_values = range(1, 9);

    $form['advanced_settings_wrapper']['menu_depth'] = [

      '#type' => 'select',

      '#title' => $this->t('Menu depth'),

      '#description' => $this->t('Maximum depth of displayed nested menu items.'),

      '#default_value' => $config->get('menu_depth'),

      '#options' => array_combine($depth_values, $depth_values),

    ];

    return parent::buildForm($form, $form_state);

  }