Render navigation block if theme requests it

gutenberg.diff

diff --git a/lib/compat.php b/lib/compat.php
index 5941bbb1f2..c0e7150384 100644
--- a/lib/compat.php
+++ b/lib/compat.php
@@ -640,6 +640,69 @@ function gutenberg_output_html_nav_menu_item( $item_output, $item, $depth, $args
 }
 add_filter( 'walker_nav_menu_start_el', 'gutenberg_output_html_nav_menu_item', 10, 4 );
 
+function gutenberg_capture_menu_items( $menu_items, $args ) {
+	$args->menu_items = $menu_items;
+	return $menu_items;
+}
+add_filter( 'wp_nav_menu_objects', 'gutenberg_capture_menu_items', 10, 2 );
+
+function gutenberg_convert_menu_item_to_block( $menu_item, &$menu_items_by_parent_id ) {
+	if ( 'html' === $menu_item->type ) {
+		return parse_blocks( $menu_item->content )[0];
+	}
+
+	$inner_blocks = array();
+
+	if ( isset( $menu_items_by_parent_id[ $menu_item->ID ] ) ) {
+		foreach ( $menu_items_by_parent_id[ $menu_item->ID ] as $child_menu_item ) {
+			$inner_blocks[] = gutenberg_convert_menu_item_to_block(
+				$child_menu_item,
+				$menu_items_by_parent_id
+			);
+		}
+	}
+
+	return array(
+		'blockName'   => 'core/navigation-link',
+		'attrs'       => array(
+			'label' => $menu_item->title,
+			'url'   => $menu_item->url,
+		),
+		'innerBlocks' => $inner_blocks,
+	);
+}
+
+function gutenberg_handle_block_nav_menu( $nav_menu, $args ) {
+	if ( ! current_theme_supports( 'block-nav-menus' ) ) {
+		return $nav_menu;
+	}
+
+	$menu_items_by_parent_id = array();
+	foreach ( $args->menu_items as $menu_item ) {
+		$menu_items_by_parent_id[ intval( $menu_item->menu_item_parent ) ][] = $menu_item;
+	}
+
+	$inner_blocks = array();
+
+	if ( isset( $menu_items_by_parent_id[0] ) ) {
+		foreach ( $menu_items_by_parent_id[0] as $menu_item ) {
+			$inner_blocks[] = gutenberg_convert_menu_item_to_block(
+				$menu_item,
+				$menu_items_by_parent_id
+			);
+		}
+	}
+
+	$navigation_block = array(
+		'blockName'   => 'core/navigation',
+		'attrs'       => array(),
+		'innerBlocks' => $inner_blocks,
+	);
+
+	return render_block( $navigation_block );
+}
+add_filter( 'wp_nav_menu', 'gutenberg_handle_block_nav_menu', 10, 2 );
+
 /**
  * Amends the paths to preload when initializing edit post.
  *

wordpress-develop.diff

Index: src/wp-content/themes/twentytwenty/functions.php
===================================================================
--- src/wp-content/themes/twentytwenty/functions.php	(revision 48655)
+++ src/wp-content/themes/twentytwenty/functions.php	(working copy)
@@ -142,6 +142,7 @@
 	$loader = new TwentyTwenty_Script_Loader();
 	add_filter( 'script_loader_tag', array( $loader, 'filter_script_loader_tag' ), 10, 2 );
 
+	add_theme_support( 'block-nav-menus' );
 }
 
 add_action( 'after_setup_theme', 'twentytwenty_theme_support' );
Index: src/wp-content/themes/twentytwenty/header.php
===================================================================
--- src/wp-content/themes/twentytwenty/header.php	(revision 48655)
+++ src/wp-content/themes/twentytwenty/header.php	(working copy)
@@ -83,25 +83,25 @@
 
 					<?php
 					if ( has_nav_menu( 'primary' ) || ! has_nav_menu( 'expanded' ) ) {
-						?>
 
-							<nav class="primary-menu-wrapper" aria-label="<?php esc_attr_e( 'Horizontal', 'twentytwenty' ); ?>" role="navigation">
+						if ( has_nav_menu( 'primary' ) ) {
 
-								<ul class="primary-menu reset-list-style">
+							wp_nav_menu(
+								array(
+									'container'      => '',
+									'items_wrap'     => '%3$s',
+									'theme_location' => 'primary',
+								)
+							);
 
-								<?php
-								if ( has_nav_menu( 'primary' ) ) {
+						} elseif ( ! has_nav_menu( 'expanded' ) ) {
+							?>
 
-									wp_nav_menu(
-										array(
-											'container'  => '',
-											'items_wrap' => '%3$s',
-											'theme_location' => 'primary',
-										)
-									);
+							<nav class="primary-menu-wrapper" aria-label="<?php esc_attr_e( 'Horizontal', 'twentytwenty' ); ?>" role="navigation">
 
-								} elseif ( ! has_nav_menu( 'expanded' ) ) {
+								<ul class="primary-menu reset-list-style">
 
+									<?php
 									wp_list_pages(
 										array(
 											'match_menu_classes' => true,
@@ -110,15 +110,14 @@
 											'walker'   => new TwentyTwenty_Walker_Page(),
 										)
 									);
+									?>
 
-								}
-								?>
-
 								</ul>
 
 							</nav><!-- .primary-menu-wrapper -->
 
-						<?php
+							<?php
+						}
 					}
 
 					if ( true === $enable_header_search || has_nav_menu( 'expanded' ) ) {