Moves all pages of a CPT (Custom Post Type) into one menu item

wp_compact_post_types.php

<?php

/**
 * Moves all pages of a CPT (Custom Post Type) into one menu item
 * Useful when a CPT is a submenu & the Add New page goes walkies
 *
 * Example: You have added a new CPT under an existing CPT or
 * menu item. When you click "Add New" for the new CPT, the URL
 * doesn't match a menu item so nothing is selected as the
 * current page. This function makes such pages show as the main
 * CPT page in the menu when selected.
 *
 * @param string|array $post_types Post Types to compact
 */
function compact_post_types( $post_types ) {
	if ( !is_array( $post_types ) ) $post_types = [ $post_types ]; 
	add_filter( 'parent_file', function( $parent_file ) use ( $post_types ) {
		foreach ( $post_types as $post_type ) {
			if ( self::get_post_type() == $post_type ) {
				$GLOBALS['submenu_file'] = 'edit.php?post_type=' . $post_type;
				return $parent_file;
			}
		}
	} );
}