New Twiddle

components.app-activity-tree.js

import Ember from 'ember';

export default Ember.Component.extend({
  tagName: 'ul',
  display: Ember.computed('data.[]', function() {
    return this.get('data').slice(-1)[0];
  }),
  nested: Ember.computed('data.[]', function() {
    return this.get('data').slice(0, -1);
  }),
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  reversedList: [{id: 21}, {id: 16}, {id: 1}],
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{app-activity-tree data=reversedList}}
<br>
<br>

templates.components.app-activity-tree.hbs

<li>
  {{display.id}}
  {{#if nested}}
      {{app-activity-tree data=nested}}
  {{/if}}
</li>

twiddle.json

{
  "version": "0.10.4",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.7.0",
    "ember-data": "2.7.0",
    "ember-template-compiler": "2.7.0"
  },
  "addons": {}
}