Source

File: wp-includes/class-wp-customize-manager.php

public function customize_preview_init() {

		/*
		 * Now that Customizer previews are loaded into iframes via GET requests
		 * and natural URLs with transaction UUIDs added, we need to ensure that
		 * the responses are never cached by proxies. In practice, this will not
		 * be needed if the user is logged-in anyway. But if anonymous access is
		 * allowed then the auth cookies would not be sent and WordPress would
		 * not send no-cache headers by default.
		 */
		if ( ! headers_sent() ) {
			nocache_headers();
			header( 'X-Robots: noindex, nofollow, noarchive' );
		}
		add_action( 'wp_head', 'wp_no_robots' );
		add_filter( 'wp_headers', array( $this, 'filter_iframe_security_headers' ) );

		/*
		 * If preview is being served inside the customizer preview iframe, and
		 * if the user doesn't have customize capability, then it is assumed
		 * that the user's session has expired and they need to re-authenticate.
		 */
		if ( $this->messenger_channel && ! current_user_can( 'customize' ) ) {
			$this->wp_die(
				-1,
				sprintf(
					/* translators: %s: customize_messenger_channel */
					__( 'Unauthorized. You may remove the %s param to preview as frontend.' ),
					'<code>customize_messenger_channel<code>'
				)
			);
			return;
		}

		$this->prepare_controls();

		add_filter( 'wp_redirect', array( $this, 'add_state_query_params' ) );

		wp_enqueue_script( 'customize-preview' );
		wp_enqueue_style( 'customize-preview' );
		add_action( 'wp_head', array( $this, 'customize_preview_loading_style' ) );
		add_action( 'wp_head', array( $this, 'remove_frameless_preview_messenger_channel' ) );
		add_action( 'wp_footer', array( $this, 'customize_preview_settings' ), 20 );
		add_filter( 'get_edit_post_link', '__return_empty_string' );

		/**
		 * Fires once the Customizer preview has initialized and JavaScript
		 * settings have been printed.
		 *
		 * @since 3.4.0
		 *
		 * @param WP_Customize_Manager $this WP_Customize_Manager instance.
		 */
		do_action( 'customize_preview_init', $this );
	}