Mark (highlight) post type parent as active item in Wordpress Navigation. When you visit a custom post type's single page, the parent menu item (the post type archive) isn't marked as active. This code solves it by comparing the slug of the current post type with the navigation items, and adds a class accordingly.

mark-parent-navigation-active.php

<?php

add_action('nav_menu_css_class', function ($classes, $item) {
		
	// Getting the current post details
	$post = get_queried_object();

	if (isset($post->post_type)) {
		if ($post->post_type == 'post') {
			$current_post_type_slug = get_permalink(get_option('page_for_posts'));
		} else {
			// Getting the post type of the current post
			$current_post_type = get_post_type_object(get_post_type($post->ID));
			$current_post_type_slug = $current_post_type->rewrite['slug'];
		}
				
		// Getting the URL of the menu item
		$menu_slug = strtolower(trim($item->url));
			
		// If the menu item URL contains the current post types slug add the current-menu-item class
		if (strpos($menu_slug, $current_post_type_slug) !== false) {
			$classes[] = 'current-menu-item';
		}
	}

	// Return the corrected set of classes to be added to the menu item
	return $classes;
}, 10, 2);

Hi Mark,
just one more thing. Your code works very well thank you. It keeps menu items active even if I have Blog > posts selected. It however does not keep a menu item active if I set up a menu Blog Category > Post.

Ie: if I set up a blog category > the category is listed and then a post item is selected from this it tends not to work.

Are you aware of this ? do you have a work around by any chance ?

thanks again for your help.

No, sorry.