Do you know CSS?

Use specific class fot it, e.g.:

.elgg-menu-page li.elgg-state-selected > a

@rivervanrain I think the problem is that the menu item doesn't have the .elgg-state-selected in the first place.

Well, @Stephen then add this class in your menu item:

      elgg_register_menu_item('page', array(
            ................

           'link_class' =>  'elgg-state-selected',
           ................
            ));

@rivervanrain After that it would be always highlighted no matter which page/tool (blog, file,etc) is being viewed. :)

@ChiswickMe I think the only way is be to register a handler for 'register', 'menu:site' hook and then use elgg_get_context() inside the handler to see whether the menu item should be set as selected using $menu_item->setSelected(true); (By "top" menu you mean the main menu, right? Or the topbar menu?)

It's not the best option to use page context inside the hook handler but I don't think there currently is any better solution.

(Btw, the same happens with the budled plugins also so the fault is not really in your plugin.)

@Juho Jaakkola :) There is another CSS trick.

I meant create CSS style and add link class into  menu:item.

In css.php

.elgg-menu-page li.elgg-state-selected > a {
    color: white;
}

.elgg-menu-page li > a.elgg-state-selected {
    color: black;
}

Sure, it's works for menu:page only.

For other items need to create similar styles too, e.g.:

.elgg-menu-extras li > a.elgg-state-selected {
    color: white;
}
.elgg-menu-extras li.elgg-state-selected > a {
    color: black;
}

Make all your plugin pages push a context like "vendor_myplugin". Give your site menu item the same name.

Elgg has a built-in menu handler that will auto-select the item based on current (topmost) context.

Thanks for the answers everyone.

For clarity, if anyone else has the same question in the future:

My menu item was setup using

$item = new ElggMenuItem('item_name', elgg_echo('Menu Label'), 'plugin/link');
elgg_register_menu_item('site', $item);

Then on each plugin page where I want to highlight that menu item I used

elgg_push_context('item_name');

Seems to work anyway!