Enable WordPress Customizer Preview on Mapped Domain

README.md

Enable WordPress Customizer Preview on Mapped Domain

If you are using the WordPress MU Domain Mapping plugin to map domains on your WordPress Multisite, the preview feature in the WordPress Customizer will not function. To resolve this issue, save enable-customizer-preview-mapped-domain.php, as must-use plugin, in wp-content/mu-plugins.

enable-customizer-preview-mapped-domain.php

<?php
/**
 * Plugin Name: Enable WordPress Customizer Preview on Mapped Domain.
 *
 */
/**
 * Prevents redirection to mapped domain when the WordPress Customizer is active.
 *
 * When WordPress Domain Mapping is active, it typically redirects all requests
 * to the mapped domain. This code modifies that behavior to allow access to
 * the Customizer on non-mapped domains by:
 * 
 * 1. Checking if the WP_Customize_Manager class exists
 * 2. Checking if domain mapping functionality is active (via redirect_to_mapped_domain function)
 * 3. Removing the standard domain mapping redirect action
 * 4. Adding a modified redirect action that doesn't redirect when Customizer preview is being loaded
 *
 * This allows administrators to use the Customizer on the original domain while
 * maintaining normal domain mapping functionality for all other requests.
 */
add_action( 'plugins_loaded', function () {

	// The WP_Customize_Manager class is loaded when the Customizer is active
	// This is important to ensure that the Customizer is available before we 
	// attempt to modify its behavior
	if ( ! class_exists( 'WP_Customize_Manager' ) ) {
		return;
	}

	// Check if the domain mapping functionality is active
	if ( function_exists( 'redirect_to_mapped_domain' ) ) {
		// Remove the standard domain mapping redirect
		remove_action( 'template_redirect', 'redirect_to_mapped_domain' );

		// Add a custom redirect that excludes Customizer preview requests
		add_action( 'template_redirect', function () {
			// Check if this is a Customizer preview request by examining query parameters
			// The customize_theme parameter indicates Customizer is active
			// The customize_messenger_channel with 'preview-0' value is specific to preview mode
			if ( isset( $_GET[ 'customize_theme' ] ) &&
				isset( $_GET[ 'customize_messenger_channel' ] ) &&
				$_GET[ 'customize_messenger_channel' ] == 'preview-0' ) {
				return; // Skip redirection for Customizer preview
			}

			// For all other requests, perform the normal domain mapping redirect
			redirect_to_mapped_domain();
		} );
	}
} );