@@ -0,0 +1,78 @@
+<?php
+/**
+ * Interactivity API: Experimental full-page client-side navigation.
+ *
+ * @package    Gutenberg
+ * @subpackage Interactivity API
+ */
+
+if ( ! class_exists( 'Gutenberg_Interactivity_API_Full_Page_Navigation' ) ) {
+
+	/**
+	 * Class Gutenberg_Interactivity_API_Full_Page_Navigation.
+	 */
+	class Gutenberg_Interactivity_API_Full_Page_Navigation {
+
+		private static $instance = null;
+
+		public static function instance() {
+			if ( null === self::$instance ) {
+				self::$instance = new Gutenberg_Interactivity_API_Full_Page_Navigation();
+			}
+			return self::$instance;
+		}
+
+		public function __construct() {
+			add_action( 'wp_head', array( $this, 'buffer_start' ) );
+			add_action( 'wp_footer', array( $this, 'buffer_end' ), 8 );
+			add_action( 'wp_enqueue_scripts', array( $this, 'enqueue_script_modules' ) );
+		}
+
+		/**
+		 * Enqueues the required script modules.
+		 */
+		public function enqueue_script_modules() {
+			wp_enqueue_script_module(
+				'@wordpress/interactivity-router/full-page'
+			);
+		}
+
+		/**
+		 * Starts output buffering at the end of the 'wp_head' action, adding the
+		 * required directives for client-side navigation to the BODY tags when the
+		 * buffer is flushed.
+		 */
+		public function buffer_start() {
+			ob_start( array( $this, 'add_directives_to_body' ) );
+		}
+
+		/**
+		 * Flushes the output buffer at the end of the 'wp_footer' action.
+		 */
+		public function buffer_end() {
+			ob_end_flush();
+		}
+
+		/**
+		 * Adds client-side navigation directives to BODY tag in the passed output
+		 * buffer.
+		 *
+		 * Note: This should probably be done per site, not by default when this option
+		 * is enabled.
+		 *
+		 * @param string $buffer Passed output buffer.
+		 *
+		 * @return string The same HTML with modified BODY attributes.
+		 */
+		public function add_directives_to_body( $buffer ) {
+			$p = new WP_HTML_Tag_Processor( $buffer );
+			if ( $p->next_tag( array( 'tag_name' => 'BODY' ) ) ) {
+				$p->set_attribute( 'data-wp-interactive', true );
+				$p->set_attribute( 'data-wp-router-region', 'core/body' );
+				return $p->get_updated_html();
+			} else {
+				return $buffer;
+			}
+		}
+	}
+}
@@ -223,6 +223,18 @@ function gutenberg_initialize_experiments_settings() {
 		)
 	);
 
+	add_settings_field(
+		'gutenberg-full-page-client-side-navigation',
+		__( 'Interactivity API: Full-page client-side navigation', 'gutenberg' ),
+		'gutenberg_display_experiment_field',
+		'gutenberg-experiments',
+		'gutenberg_experiments_section',
+		array(
+			'label' => __( 'Enables full-page client-side navigation, powered by the Interactivity API.', 'gutenberg' ),
+			'id'    => 'gutenberg-full-page-client-side-navigation',
+		)
+	);
+
 	register_setting(
 		'gutenberg-experiments',
 		'gutenberg-experiments'
 
@@ -156,3 +156,9 @@ function gutenberg_is_experiment_enabled( $name ) {
 if ( gutenberg_is_experiment_enabled( 'gutenberg-media-processing' ) ) {
 	require_once __DIR__ . '/experimental/media/load.php';
 }
+
+// Interactivity API full-page client-side navigation.
+if ( gutenberg_is_experiment_enabled( 'gutenberg-full-page-client-side-navigation' ) ) {
+	require __DIR__ . '/experimental/interactivity-api/class-gutenberg-interactivity-api-full-page-navigation.php';
+	Gutenberg_Interactivity_API_Full_Page_Navigation::instance();
+}
@@ -25,7 +25,10 @@ function () {
 		 * required to ensure that client hydration works even when the rendered
 		 * HTML contains unbalanced tags and it couldn't be processed in the server.
 		 */
-		if ( 'true' === $_GET['disable_server_directive_processing'] ) {
+		if (
+			isset( $_GET['disable_server_directive_processing'] ) &&
+			'true' === $_GET['disable_server_directive_processing']
+		) {
 			// Ensure the interactivity API is loaded.
 			wp_interactivity();
 			// But remove the server directive processing.
 
@@ -0,0 +1,15 @@
+{
+	"$schema": "https://schemas.wp.org/trunk/block.json",
+	"apiVersion": 2,
+	"name": "test/router-script-modules-alpha",
+	"title": "E2E Interactivity tests - router modules - Alpha",
+	"category": "text",
+	"icon": "heart",
+	"description": "",
+	"supports": {
+		"interactivity": true
+	},
+	"textdomain": "e2e-interactivity",
+	"viewScriptModule": "file:./view.js",
+	"render": "file:./render.php"
+}
@@ -0,0 +1 @@
+export default 'alpha';
@@ -0,0 +1,27 @@
+<?php
+/**
+ * HTML for testing the iAPI's script module assets management.
+ *
+ * @package gutenberg-test-interactive-blocks
+ *
+ * @phpcs:disable VariableAnalysis.CodeAnalysis.VariableAnalysis.UndefinedVariable
+ */
+
+$module_path = '/module.js';
+$module_url  = plugins_url( $module_path, __FILE__ );
+wp_register_script_module(
+	'test/router-script-modules-alpha',
+	$module_url,
+	array(),
+	filemtime( plugin_dir_path( __FILE__ ) . $module_path )
+);
+
+$wrapper_attributes = get_block_wrapper_attributes(
+	array(
+		'data-testid'         => 'alpha-block',
+		'data-wp-interactive' => 'test/router-script-modules-alpha',
+		'data-wp-text'        => 'state.name',
+	)
+);
+?>
+<p <?php echo $wrapper_attributes; ?>></p>
@@ -0,0 +1,6 @@
+<?php return array(
+	'dependencies' => array(
+		'@wordpress/interactivity',
+		'test/router-script-modules-alpha',
+	),
+);
@@ -0,0 +1,19 @@
+/**
+ * WordPress dependencies
+ */
+import { store } from '@wordpress/interactivity';
+
+/**
+ * External dependencies
+ */
+// eslint-disable-next-line import/no-unresolved
+import name from 'test/router-script-modules-alpha';
+
+store( 'test/router-script-modules-alpha', {
+	state: {
+		name,
+	},
+} );
+
+const { actions } = store( 'test/router-script-modules' );
+actions.pushName?.( name );