Multilevel menu with only expanded current branch – how?

Thanks oorafal for your response in the other thread. I’m replying here since this is a more general forum thread instead of one under a specific plugin.
I ended up doing something similar but on the server side by creating my own menu widget plugin. I iterate through the selected menu and add a hidden class to all the menu-items which either do not have the current page as its menu_item_parent or do not have a menu_item_parent matching any of the current page’s ancestors.
It’s just one PHP file. Here is the full code if it helps anyone else out in the future.

<?php
/*
Plugin Name: OSN Pages Menu
Plugin URI: https://www.opensciencenetwork.org/
Description: A simple widget to display the side menu for the OSN website
Version: 1.0
Author: Sean Murphy
Author URI: https://www.betterourtomorrow.com/
License: GPL2
*/
class OSN_Pages_Menu extends WP_Widget {

	// constructor
	function OSN_Pages_Menu() {
		parent::WP_Widget(false, $name = __('OSN Pages Menu', 'OSN_Pages_Menu') );
	}

	// widget form creation
	function form($instance) {
		//get menus...
		$menus = wp_get_nav_menus( array( 'orderby' => 'name' ) );
		$noitems = true;
		if( !empty( $menus ) ){
			foreach( $menus as $i=>$menu ){
				$menus[ $i ]->_items = wp_get_nav_menu_items( $menu->term_id );
				if( !empty( $menus[ $i ]->_items ) ){
					$noitems = false;
				}
			}
		}

		//no menus exist with items...
		if( $noitems ){
			echo '<p>'. sprintf( __('There are no populated menus. <a href="%s">Go to Menus</a>.'), admin_url('nav-menus.php') ) .'</p>';
			return;
		}
?>
		<p>
			<label for="<?php echo $this->get_field_id('menu'); ?>"><?php _e('Select Menu:'); ?></label>
			<select id="<?php echo $this->get_field_id('menu'); ?>"
					class="widget-<?php echo $this->id_base; ?>-selectmenu widget-<?php echo $this->id_base; ?>-listen"
					name="<?php echo $this->get_field_name('menu'); ?>">
<?php
		foreach( $menus as $i=>$menu ){
			if( !empty( $menu->_items ) ){
?>
				<option <?php selected($instance['menu'], $menu->term_id); ?> value="<?php echo $menu->term_id; ?>"><?php echo $menu->name; ?></option>
<?php
			}
		}
?>
			</select>
		</p>
<?php
	}

	// widget update
	function update($new_instance, $old_instance) {
		$instance = $old_instance;
		// Fields
		$instance['menu'] = strip_tags($new_instance['menu']);
		return $instance;
	}

	// widget display
	function widget($args, $instance) {
		extract( $args );
		// create the widget...
		if( !empty($instance['menu'] ) ){
			$menu = wp_get_nav_menu_object( intval($instance['menu']) );
			//no menu, no output...
			if ( !empty( $menu ) ){

				$items = wp_get_nav_menu_items( $menu->term_id, array( 'order' => 'DESC' ));
				$item_arrays = (array)$items[0];

				$this_menu_item_parent = -1;
				$this_db_id = -1;
				$top_db_id = $items[0]->db_id;//first menu item will always be a top parent
				$top_menu_item_parent = $items[0]->menu_item_parent;//parent of first item will be the same parent of all top level item
				$exclude_array = array();
				foreach($items as $item) {
					if($item->object_id == get_the_ID()) {
						$this_menu_item_parent = $item->menu_item_parent;
						$this_db_id = $item->db_id;
					}
					$exclude_array[] = $item->db_id; //adds all menu ids to the array to be removed later
				}
				if($this_menu_item_parent == -1) { //means there isn't a menu item for this page
					$this_menu_item_parent = $top_menu_item_parent;
					$this_db_id = $top_db_id;
				}

				echo $before_widget;
				// Display the widget
				echo '<div class="side_nav widget rounded_2">';

				$next_menu_item_parent = $this_db_id;//this accounts for all the children of the current menu item
				while($next_menu_item_parent != -1) {
					$current_menu_item_parent = $next_menu_item_parent;
					foreach($items as $item) {
						if($item->db_id == $current_menu_item_parent) {
							$next_menu_item_parent = $item->menu_item_parent;
						}
						if($item->menu_item_parent == $current_menu_item_parent) {
							$exclude_array = array_diff($exclude_array, array($item->db_id));//removes the pages we want from the list of excluded pages
						}
					}
					if($current_menu_item_parent == $top_menu_item_parent) {
						$next_menu_item_parent = -1;
					}
				}
				$menu = wp_nav_menu( array('menu' => intval($instance['menu']), 'echo' => 0 ));
				foreach($exclude_array as $excluded_id) {
					$menu = str_replace('menu-item-'.$excluded_id, 'menu-item-'.$excluded_id.' hidden', $menu);
				}
				echo $menu;

				echo '</div>';
				echo $after_widget;
			}
		}
	}
}

// register widget
add_action('widgets_init', create_function('', 'return register_widget("OSN_Pages_Menu");'));
?>