https://stackoverflow.com/questions/45657041/ember-willtransition-in-route-breaks-link-to-helper-active-class

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember 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('parent', function(){
		this.route('child-a');
    this.route('child-b');
  })
});

export default Router;

routes.parent.child-a.js

import Ember from 'ember';

export default Ember.Route.extend({
});

routes.parent.child-b.js

import Ember from 'ember';

export default Ember.Route.extend({
});

routes.parent.js

import Ember from 'ember';

export default Ember.Route.extend({
	actions: {
  	willTransition() {
  		this.refresh();
  	}
  }
});

styles.app.css

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

a {
  color: inherit;
}

a.active {
	color: red;
}

templates.application.hbs

<h1>/application</h1>

{{link-to 'Go to /parent' 'parent'}}

{{outlet}}

templates.parent.hbs

<h2>/parent</h2>

<p>When you click one of this links, all of it's siblings are styled as active, even if they produce different dynamic segments.</p>

{{link-to 'Child A' 'parent.child-a'}}
{{link-to 'Child B' 'parent.child-b'}}

<p>If you go to routes/parent and remove the snippet 'willTransition' it will work prefectly: each link will be active when they actually are equal to the dynamic segment shown in the URL</p>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}