function twentynineteen_add_dropdown_icons( $output, $item, $depth, $args ) {

  // Only add class to 'top level' items on the 'primary' menu.
  if ( ! isset( $args->theme_location ) || 'menu-1' !== $args->theme_location ) {
    return $output;
  }

  if ( in_array( 'mobile-parent-nav-menu-item', $item->classes, true ) && isset( $item->original_id ) ) {
    // Inject the keyboard_arrow_left SVG inside the parent nav menu item, and let the item link to the parent item.
    // @todo Only do this for nested submenus? If on a first-level submenu, then really the link could be "#" since the desire is to remove the target entirely.
    $link = sprintf(
      '<button class="menu-item-link-return" tabindex="-1">%s',
      twentynineteen_get_icon_svg( 'chevron_left', 24 )
    );

    // replace opening <a> with <button>
    $output = preg_replace(
      '/<a\s.*?>/',
      $link,
      $output,
      1 // Limit.
    );

    // replace closing </a> with </button>
    $output = preg_replace(
      '#</a>#i',
      '</button>',
      $output,
      1 // Limit.
    );

  } elseif ( in_array( 'menu-item-has-children', $item->classes, true ) ) {

    // Add SVG icon to parent items.
    $icon = twentynineteen_get_icon_svg( 'keyboard_arrow_down', 24 );

    $output .= sprintf(
      '<button class="submenu-expand" tabindex="-1">%s</button>',
      $icon
    );
  }

  return $output;
}