function _wp_customize_include() {

	$is_customize_admin_page = ( is_admin() && 'customize.php' == basename( $_SERVER['PHP_SELF'] ) );
	$should_include          = (
		$is_customize_admin_page
		||
		( isset( $_REQUEST['wp_customize'] ) && 'on' == $_REQUEST['wp_customize'] )
		||
		( ! empty( $_GET['customize_changeset_uuid'] ) || ! empty( $_POST['customize_changeset_uuid'] ) )
	);

	if ( ! $should_include ) {
		return;
	}

	/*
	 * Note that wp_unslash() is not being used on the input vars because it is
	 * called before wp_magic_quotes() gets called. Besides this fact, none of
	 * the values should contain any characters needing slashes anyway.
	 */
	$keys       = array( 'changeset_uuid', 'customize_changeset_uuid', 'customize_theme', 'theme', 'customize_messenger_channel', 'customize_autosaved' );
	$input_vars = array_merge(
		wp_array_slice_assoc( $_GET, $keys ),
		wp_array_slice_assoc( $_POST, $keys )
	);

	$theme             = null;
	$changeset_uuid    = false; // Value false indicates UUID should be determined after_setup_theme to either re-use existing saved changeset or else generate a new UUID if none exists.
	$messenger_channel = null;
	$autosaved         = null;
	$branching         = false; // Set initially fo false since defaults to true for back-compat; can be overridden via the customize_changeset_branching filter.

	if ( $is_customize_admin_page && isset( $input_vars['changeset_uuid'] ) ) {
		$changeset_uuid = sanitize_key( $input_vars['changeset_uuid'] );
	} elseif ( ! empty( $input_vars['customize_changeset_uuid'] ) ) {
		$changeset_uuid = sanitize_key( $input_vars['customize_changeset_uuid'] );
	}

	// Note that theme will be sanitized via WP_Theme.
	if ( $is_customize_admin_page && isset( $input_vars['theme'] ) ) {
		$theme = $input_vars['theme'];
	} elseif ( isset( $input_vars['customize_theme'] ) ) {
		$theme = $input_vars['customize_theme'];
	}

	if ( ! empty( $input_vars['customize_autosaved'] ) ) {
		$autosaved = true;
	}

	if ( isset( $input_vars['customize_messenger_channel'] ) ) {
		$messenger_channel = sanitize_key( $input_vars['customize_messenger_channel'] );
	}

	/*
	 * Note that settings must be previewed even outside the customizer preview
	 * and also in the customizer pane itself. This is to enable loading an existing
	 * changeset into the customizer. Previewing the settings only has to be prevented
	 * here in the case of a customize_save action because this will cause WP to think
	 * there is nothing changed that needs to be saved.
	 */
	$is_customize_save_action = (
		wp_doing_ajax()
		&&
		isset( $_REQUEST['action'] )
		&&
		'customize_save' === wp_unslash( $_REQUEST['action'] )
	);
	$settings_previewed       = ! $is_customize_save_action;

	require_once ABSPATH . WPINC . '/class-wp-customize-manager.php';
	$GLOBALS['wp_customize'] = new WP_Customize_Manager( compact( 'changeset_uuid', 'theme', 'messenger_channel', 'settings_previewed', 'autosaved', 'branching' ) );
}