New Twiddle

router.js

import Ember from 'ember';
import config from './config/environment';

const Router = Ember.Router.extend({
  location: 'none',
  rootURL: config.rootURL
});

Router.map(function() {
  this.route('foo', { path: '/foo/:topId' }, function() {
    this.route('nested', { path: '/nested/:nestedId' });
  });
  
  this.route('bar', function() {
    this.route('nested');
  });
});

Ember.Route.reopen({
  setupController() {
    this._super(...arguments);
    this.controllerFor('application')
      .set('routeName', this.get('routeName'));
  }
});

export default Router;

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

a.active {
  font-weight: bold; 
}

templates.application.hbs

Current route: {{routeName}}

<br><br>

{{!
  I have specified that the link to 'foo.index'
  should be considered *current* when on 'foo.nested'
  yet it does not receive the active class name, but
  this works fine without route params.
}}

Does not work with params:

{{#link-to 'foo.index' 1 current-when='foo.index foo.nested'~}}
  foo.index
{{~/link-to}} |

{{#link-to 'foo.nested' 1 2~}}
  foo.nested
{{~/link-to}}

<br><br>

Works without params:

{{#link-to 'bar.index' current-when='bar.index bar.nested'~}}
  bar.index
{{~/link-to}} |

{{#link-to 'bar.nested'~}}
  bar.nested
{{~/link-to}}

<hr>

{{outlet}}

templates.bar.hbs

Bar

<br><br>

{{outlet}}

templates.bar.nested.hbs

-> Nested

<br><br>

{{outlet}}

templates.foo.hbs

Foo {{model.topId}}

<br><br>

{{outlet}}

templates.foo.nested.hbs

-> Nested {{model.nestedId}}

<br><br>

{{outlet}}

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.9.0",
    "ember-data": "2.9.0",
    "ember-template-compiler": "2.9.0",
    "ember-testing": "2.9.0"
  },
  "addons": {}
}