Fix WordPress Author dropdown in multi-role set-up

fix-author-dropdown.php

<?php
/**
 * If your website has custom roles and capabilities, and post-types with custom edit_posts capabilities,
 * the dropdown on the _overview_ page, _quick edit_ is CORRECT.
 * however, the dropdown on the _create new post_ or _edit existing post_ pages is NOT.
 * This code fixes that.
 *
 * @package    FixesByRemonPel
 * @subpackage FixesByRemonPel/RestUserQuery
 */

add_filter(
	'rest_user_query',
	function ( $prepared_args ) {
		// If asking for deprecated `who` parameter, and referer is set, try to determine the post type and set the capability.
		if ( ! empty( $prepared_args['who'] ) && ! empty( $_SERVER['HTTP_REFERER'] ) ) {
			$referer_url = parse_url( $_SERVER['HTTP_REFERER'] );
			parse_str( $referer_url['query'], $referer_query );
			// We successfully parsed the query string.
			if ( $referer_query ) {
				// We either have a post or a post_type.
				$post_id   = $referer_query['post'] ?? 0;
				$post_type = $referer_query['post_type'] ?? '';
				if ( $post_id || $post_type ) {
					// If we have a post_id, we can determine the post_type.
					$post_type = $post_type ?: get_post_type( $post_id );
					// If we have a post_type, we can determine the capability.
					if ( $post_type ) {
						$post_type_object = get_post_type_object( $post_type );
						$capability       = $post_type_object->cap->edit_posts;
						// If we have a capability, we can replace the `who` argument with the `capability` argument.
						if ( $capability ) {
							$prepared_args['capability'] = $capability;
							unset( $prepared_args['who'] );
						}
					}
				}
			}
		}

		return $prepared_args;
	}
);