Bug fixes in functions.php

functions.php

<?php
/**
 * functions.php
 */

/**
 * Fix a long-standing issue with ACF, where fields sometimes aren't shown
 * in previews (ie. from Preview > Open in new tab).
 *
 * @link https://support.advancedcustomfields.com/forums/topic/custom-fields-on-post-preview/
 */
if ( class_exists( 'acf_revisions' ) ) {
  $acf_revs_cls = acf()->revisions;
  remove_filter( 'acf/validate_post_id', array( $acf_revs_cls, 'acf_validate_post_id', 10 ) );
}

/**
 * Ensure ACF fields can be previewed when post revisions are disabled
 *
 * @link https://gist.github.com/mcaskill/3812b96b7795c3d31c36ad542f454ce9
 */
function bc_fix_acf_post_revisions( int $num, WP_Post $post ) {
  if ( WP_POST_REVISIONS ) {
    return $num;
  }

  if ( !post_type_supports( $post->post_type, 'revisions' ) ) {
    return $num;
  }

  if ( !is_preview() || empty( $_GET['preview_id'] ) ) {
    return $num;
  }

  $preview_id = (int) $_GET['preview_id'];

  if ( $preview_id !== $post->ID ) {
    return $num;
  }

  if ( !doing_filter( 'wp_revisions_to_keep' ) || !doing_filter( 'acf/validate_post_id' ) ) {
    return $num;
  }

  return 1;
}
add_filter( 'wp_revisions_to_keep', 'bc_fix_acf_post_revisions', 10, 2 );