Prepend parent page title to page list in the menu builder

prepend-page-parent-nav-item.php

<?php

add_action( 'admin_head-nav-menus.php', 'enable_menu_page_selector_filter' );

function enable_menu_page_selector_filter() {
	// Unfortunately Walker_Nav_Menu_Checklist relies on a filter that
	// is used also elsewhere and can't be filtered reliably.
	add_filter( 'the_title', 'prepend_page_parent', 10, 2 );
}

function prepend_page_parent( $title, $post_id ) {
	$post = get_post( $post_id );

	if ( 'page' == $post->post_type && ! empty( $post->post_parent ) ) {
		return sprintf(
				'%s → %s',
				wp_trim_words( get_the_title( $post->post_parent ), 2 ),
				$title
			);
	}

	return $title;
}