Navigation block: add location->primary to fallback nav creation for classic menus.

packages/block-library/src/navigation/edit/index.js

@@ -284,18 +284,44 @@ function Navigation( {
 			! hasResolvedNavigationMenus ||
 			isConvertingClassicMenu ||
 			fallbackNavigationMenus?.length > 0 ||
-			classicMenus?.length !== 1
+			! classicMenus?.length
 		) {
 			return;
 		}
 
 		// If there's non fallback navigation menus and
-		// only one classic menu then create a new navigation menu based on it.
-		convertClassicMenu(
-			classicMenus[ 0 ].id,
-			classicMenus[ 0 ].name,
-			'publish'
+		// a classic menu with a `primary` location or slug,
+		// then create a new navigation menu based on it.
+		// Otherwise, use the first classic menu.
+		const primaryMenus = classicMenus.filter(
+			( classicMenu ) =>
+				classicMenu.locations.includes( 'primary' ) ||
+				classicMenu.slug === 'primary'
 		);
+
+		if ( primaryMenus.length ) {
+			// Sort by location to allow the menu assigned to primary location to take precedence.
+			primaryMenus.sort( ( a, b ) => {
+				if ( a.locations < b.locations ) {
+					return 1;
+				}
+				if ( a.locations > b.locations ) {
+					return -1;
+				}
+				return 0;
+			} );
+			convertClassicMenu(
+				primaryMenus[ 0 ].id,
+				primaryMenus[ 0 ].name,
+				'publish'
+			);
+		} else {
+			convertClassicMenu(
+				classicMenus[ 0 ].id,
+				classicMenus[ 0 ].name,
+				'publish'
+			);
+		}
 	}, [ hasResolvedNavigationMenus ] );
 
 	const navRef = useRef();

packages/block-library/src/navigation/index.php

@@ -257,10 +257,26 @@ function block_core_navigation_get_classic_menu_fallback() {
 	$classic_nav_menus = wp_get_nav_menus();
 
 	// If menus exist.
-	if ( $classic_nav_menus && ! is_wp_error( $classic_nav_menus ) && count( $classic_nav_menus ) === 1 ) {
-		// Use the first classic menu only. Handles simple use case where user has a single
-		// classic menu and switches to a block theme. In future this maybe expanded to
-		// determine the most appropriate classic menu to be used based on location.
+	if ( $classic_nav_menus && ! is_wp_error( $classic_nav_menus ) ) {
+		// Handles simple use case where user has a classic menu and switches to a block theme.
+
+		// Returns the menu assigned to location `primary`.
+		$locations = get_nav_menu_locations();
+		if ( isset( $locations['primary'] ) ) {
+			$primary_menu = wp_get_nav_menu_object( $locations['primary'] );
+			if ( $primary_menu ) {
+				return $primary_menu;
+			}
+		}
+
+		// Returns a menu if `primary` is its slug.
+		foreach ( $classic_nav_menus as $classic_nav_menu ) {
+			if ( 'primary' === $classic_nav_menu->slug ) {
+				return $classic_nav_menu;
+			}
+		}
+
+		// Otherwise return the most recently created classic menu.
 		return $classic_nav_menus[0];
 	}
 }