WordPress Plugin for Customizer Cross-Domain Workaround (Trac #39128)

trac-39128-customizer-cross-domain-workaround.php

<?php
/**
 * Plugin Name: Customizer Cross-Domain Workaround (Trac #39128)
 * Description: Force customizer preview to use siteurl instead of home URL to work around Trac #39128. Temp fix for issue loading customizer preview when home and siteurl have different domains (cross-domain).
 * Plugin URI: https://core.trac.wordpress.org/ticket/39128
 * Author: Weston Ruter, XWP
 * Author URI: https://make.xwp.co/
 *
 * Copyright (c) 2017 XWP (https://xwp.co/)
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License, version 2 or, at
 * your discretion, any later version, as published by the Free
 * Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 */

namespace Trac39128;

add_action( 'customize_register', function( \WP_Customize_Manager $wp_customize ) {

	// Capture original home URL.
	$home_url = get_option( 'home' );

	// Override home URL to be the siteurl.
	add_filter( 'option_home', function() {
		return get_option( 'siteurl' );
	} );

	// Include the original home URL among the allowed URLs.
	add_filter( 'customize_allowed_urls', function( $urls ) use ( $home_url ) {
		$urls[] = $home_url;
		return $urls;
	} );

	/*
	 * Ensure that the home URL will pass the wp_validate_redirect() check in \WP_Customize_Manager::set_preview_url().
	 * This is to ensure that when clicking Customize from the frontend the URL will be accepted.
	 */
	add_filter( 'allowed_redirect_hosts', function( $hosts ) use ( $home_url ) {
		$hosts[] = parse_url( $home_url, PHP_URL_HOST );
		return $hosts;
	} );

	/*
	 * Fix up the preview URL coming from the url query param when clicking Customize
	 * on the frontend to use the siteurl instead of the home URL.
	 */
	add_action( 'customize_controls_init', function() use ( $wp_customize ) {
		$parsed_preview_url = parse_url( $wp_customize->get_preview_url() );
		$preview_url = get_option( 'siteurl' );
		if ( isset( $parsed_preview_url['path'] ) ) {
			$preview_url .= $parsed_preview_url['path'];
		} else {
			$preview_url .= '/';
		}
		if ( isset( $parsed_preview_url['query'] ) ) {
			$preview_url .= '?' . $parsed_preview_url['query'];
		}
		$wp_customize->set_preview_url( $preview_url );
	} );
} );