New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  oneSelected: Ember.computed.equal('toggled', 1),
  twoSelected: Ember.computed.equal('toggled', 2),
  threeSelected: Ember.computed.equal('toggled', 3),
  actions: {
   toggle(item) {
    this.set('toggled', item); 
   }
  }
});

templates.application.hbs

<style>
	.sizing-fun {
    width: 500px;
    height: 100px;
    text-align: center;
    line-height: 100px;
    cursor: pointer;
  }

  .item {
    border: 1px solid grey;
    border-bottom-width: 0px;
  }
  
  .item:last-child {
    border-bottom-width: 1px;
  }
  
  .item:hover {
    border-color: cornflowerblue;
  }
  .item:hover + .item {
    border-top-color: cornflowerblue;
  }
  
  .item.o__selected {
    border-color: red;
  }
  .item.o__selected + .item {
    border-top-color: red;
  }
  
  // Remove this if you want blue to take precedence on hover
  .item:hover + .item.o__selected {
    border-top-color: red;
  }
</style>

<div class="container">
  <div class="sizing-fun item {{if oneSelected 'o__selected'}}" {{action 'toggle' 1}}>
  	Item 1
  </div>
  <div class="sizing-fun item {{if twoSelected 'o__selected'}}" {{action 'toggle' 2}}>
  	Item 2
  </div>
  <div class="sizing-fun item {{if threeSelected 'o__selected'}}" {{action 'toggle' 3}}>
  	Item 3
  </div>
</div>

twiddle.json

{
  "version": "0.13.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.16.2",
    "ember-template-compiler": "2.16.2",
    "ember-testing": "2.16.2"
  },
  "addons": {
    "ember-data": "2.16.3"
  }
}