Custom dashboard menu does not stay open for the custom Taxonomy within it

Short Version:

Custom menu with a custom post type that supports a custom taxonomy. The custom post type works, the menu for it works and stays open. The custom taxonomy works, but the sidebar menu doesn’t stay open. I need to make the category keep the submenu open, as it does natively (when not placed in another menu).

Long version:

I’ve got a custom menu created with add_menu_page:

add_menu_page(
  'Directory',           // Page Title
  'Directory',           // Menu Title
  'manage_options',      // Capability
  'directory',           // Slug
  null,                  // Rendering Function
  'div'                  // Menu Icon (or "div" for CSS)
);

I have also created a custom Post Type and a custom Taxonomy. I want to make these appear within my custom menu, which according to the show_in_menu argument (See wiki, can’t post the link), this should be supported natively.

However, it simply does not work correctly. True, this option does stick the “Manage Directory” link into my custom menu, but it does not move the “Add Contact” or “Directory Categories (Custom taxonomy)” links into the menu.

Here are a few previews:

Custom Menu: http://radleygh.com/images/chrome_2013-114-14-14-16-91.png

Default Menu: http://radleygh.com/images/chrome_2013-114-14-14-53-25.png

As you can see, the Default Menu has three links. I need to move all of these to my custom men (Where I will be adding submenus).

One trick I learned awhile back is that you can create submenus for existing pages, and they will behave as if they are the real menu. At least, this works for most things. It does not work with my taxonomy. See the code below:

add_submenu_page( // Manage Directory
  'directory',
  'Manage Directory',                  // Page Title
  'Manage Directory',                          // Menu Title
  'manage_options',                    // Capability
  'edit.php?post_type=directory_post'  // Slug
);
add_submenu_page( // Add Contact
  'directory',
  'Add Contact',                            // Page Title
  'Add Contact',                            // Menu Title
  'manage_options',                         // Capability
  'post-new.php?post_type=directory_post'   // Slug
);
add_submenu_page( // Categories
  'directory',
  'Categories',                             // Page Title
  'Categories',                             // Menu Title
  'manage_options',                         // Capability
  'edit-tags.php?taxonomy=directory_category&post_type=directory_post'  // Slug
);

The above code gives me the links I want, and the “slug” for each is smart enough to point to the correct pages. In fact, it’s as if the links are the native ones. But of course, they aren’t.

The problem though, is that for some reason the “Categories” link alone does not keep the menu open. The others do.

I believe the issue is that the taxonomy does not support the “show_in_menu” argument. Even though the argument is “broken” as I explained above, it does at least keep the menu open.

Here is the definition for the directory post type and taxonomy, in full

// Register the post type and custom category (aka taxonomy)
function dir_register_directory() {
  $args = array(
    'labels' => array(
          'name' => 'Directory Listings',
          'singular_name' => 'Directory Listing',
          'add_new' => 'Add Contact',
          'add_new_item' => 'Add New Contact',
          'edit_item' => 'Edit Contact',
          'new_item' => 'New Contact',
          'all_items' => 'Manage Directory',
          'view_item' => null, // Don't need to view these
          'search_items' => 'Search Directory',
          'not_found' =>  'No contacts found',
          'not_found_in_trash' => 'No contacts found in Trash', 
          'parent_item_colon' => '',
          'menu_name' => 'Directory'
        ),
    'capability_type'     => 'post',
    'hierarchical'        => false,

    'public'              => true,
    'show_in_menu'        => 'directory', // Nest the link under custom menu
    'show_in_nav_menus'   => false, // Do NOT show these listings under Appearance > Menus

    'publicly_queryable'  => false, // Will not show up in search results
    'query_var'           => false, // Cannot access via ?{query_var}=
    'rewrite'             => false, // Entries should not be accessed by URL

    'has_archive'         => 'directory_category', // Use our custom taxonomy
    'menu_position'       => 1,  // Should be on top of the parent menu's list
    'supports'            => array(
          'page-attributes', // Used for Sort Order
        )
   );
   register_post_type( 'directory_post', $args );

  // Create a custom category section for the Directory page
  register_taxonomy(
    'directory_category',
    'directory_post',
    array(
      'rewrite' => false,
      'hierarchical' => true,
      'sort' => true,
      'public' => true,
      'show_in_nav_menus'   => false, // Do NOT show these listings under Appearance > Menus
    )
  );

  register_taxonomy_for_object_type( 'directory_category', 'directory_post' );
}
add_action('init', 'dir_register_directory');