First support for loadOnClientNavigation

src/wp-includes/blocks.php

@@ -175,15 +175,24 @@ function register_block_script_module_id( $metadata, $field_name, $index = 0 ) {
 	$block_version       = isset( $metadata['version'] ) ? $metadata['version'] : false;
 	$module_version      = isset( $module_asset['version'] ) ? $module_asset['version'] : $block_version;
 
-	// Blocks using the Interactivity API are server-side rendered, so they are by design not in the critical rendering path and should be deprioritized.
+	$supports_interactivity_true  = isset( $block_type->supports['interactivity'] ) && true === $block_type->supports['interactivity'];
+	$is_interactive               = $supports_interactivity_true || ( isset( $block_type->supports['interactivity']['interactive'] ) && true === $block_type->supports['interactivity']['interactive'] );
+	$supports_client_navigation   = $supports_interactivity_true || ( isset( $block_type->supports['interactivity']['clientNavigation'] ) && true === $block_type->supports['interactivity']['clientNavigation'] );
+
 	$args = array();
-	if (
-		( isset( $metadata['supports']['interactivity'] ) && true === $metadata['supports']['interactivity'] ) ||
-		( isset( $metadata['supports']['interactivity']['interactive'] ) && true === $metadata['supports']['interactivity']['interactive'] )
-	) {
+
+	// Blocks using the Interactivity API are server-side rendered, so they are
+	// by design not in the critical rendering path and should be deprioritized.
+	if ( $is_interactive ) {
 		$args['fetchpriority'] = 'low';
 	}
 
+	// Blocks using the Interactivity API that support client-side navigation
+	// must be marked as such in their script modules.
+	if ( $is_interactive && $supports_client_navigation ) {
+		wp_interactivity()->add_client_navigation_support_to_script_module( $module_id );
+	}
+
 	wp_register_script_module(
 		$module_id,
 		$module_uri,

src/wp-includes/interactivity-api/class-wp-interactivity-api.php

@@ -85,6 +85,14 @@
 	 */
 	private $has_processed_router_region = false;
 
+	/**
+	 * Set of script modules that can be loaded after client-side navigation.
+	 * 
+	 * @since 6.9.0
+	 * @var array<string, bool>
+	 */
+	private $script_modules_that_can_load_on_client_navigation = array();
+
 	/**
 	 * Stack of namespaces defined by `data-wp-interactive` directives, in
 	 * the order they are processed.
@@ -371,10 +379,51 @@
 	 * Adds the necessary hooks for the Interactivity API.
 	 *
 	 * @since 6.5.0
+	 * @since 6.9.0 Adds support for client-side navigation in script modules.
 	 */
 	public function add_hooks() {
 		add_filter( 'script_module_data_@wordpress/interactivity', array( $this, 'filter_script_module_interactivity_data' ) );
 		add_filter( 'script_module_data_@wordpress/interactivity-router', array( $this, 'filter_script_module_interactivity_router_data' ) );
+		add_filter( 'wp_script_attributes', array( $this, 'add_load_on_client_navigation_attribute_to_script_modules' ), 10, 1 );
+	}
+
+	/**
+	 * Adds the `data-wp-router-options` attribute to script modules that
+	 * support client-side navigation.
+	 *
+	 * This method filters the script attributes to include loading instructions
+	 * for the Interactivity API router, indicating which modules can be loaded
+	 * during client-side navigation.
+	 *
+	 * @since 6.9.0
+	 *
+	 * @param array $attributes The script tag attributes.
+	 * @return array The modified script tag attributes.
+	 */
+	public function add_load_on_client_navigation_attribute_to_script_modules( $attributes ) {
+		if (
+			isset( $attributes['type'] ) && $attributes['type'] === 'module' && isset( $attributes['id'] )
+			&& array_key_exists( $attributes['id'], $this->script_modules_that_can_load_on_client_navigation )
+		) {
+			$attributes['data-wp-router-options'] = wp_json_encode( array( 'loadOnClientNavigation' => true ) );
+		}
+		return $attributes;
+	}
+
+	/**
+	 * Marks a script module as compatible with client-side navigation.
+	 * 
+	 * This method registers a script module to be loaded during client-side
+	 * navigation in the Interactivity API router. Script modules marked with
+	 * this method will have the `loadOnClientNavigation` option enabled in the
+	 * `data-wp-router-options` directive.
+	 *
+	 * @since 6.9.0
+	 *
+	 * @param string $script_module_id The script module identifier.
+	 */
+	public function add_client_navigation_support_to_script_module( $script_module_id ) {
+		$this->script_modules_that_can_load_on_client_navigation[ $script_module_id . '-js-module' ] = true;
 	}
 
 	/**

src/wp-includes/script-modules.php

@@ -199,6 +199,11 @@ function wp_default_script_modules() {
 			$args['fetchpriority'] = 'low';
 		}
 
+		// Marks all Core blocks as compatible with client-side navigation.
+		if ( str_starts_with( $script_module_id, '@wordpress/block-library' ) ) {
+			wp_interactivity()->add_client_navigation_support_to_script_module( $script_module_id );
+		}
+
 		$path = includes_url( "js/dist/script-modules/{$file_name}" );
 		wp_register_script_module( $script_module_id, $path, $script_module_data['dependencies'], $script_module_data['version'], $args );
 	}