Extend "Homepage displays" dropdown in admin panel

<?php

/**
 * Extend "Homepage displays" dropdown in admin panel
 */
function add_pages_to_dropdown( $pages, $r ) {
  if ( ! isset( $r['name'] ) ) {
    return $pages;
  }

  if ( 'page_on_front' === $r['name'] ) {
    $args = array(
      'post_type'      => 'arts_portfolio_item',
      'posts_per_page' => -1,
    );

    $portfolios = get_posts( $args );
    $pages      = array_merge( $pages, $portfolios );
  }

  return $pages;
}
add_filter( 'get_pages', 'add_pages_to_dropdown', 10, 2 );