ember-basic-dropdown animation issue

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

styles.app.css

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


.trigger-bootstrap-feel {
  display: inline-block;
  text-align: center;
  vertical-align: middle;
  cursor: pointer;
  border: 1px solid #ccc;
  padding: 6px 12px;
  line-height: 1.42857143;
  border-radius: 4px;
  color: #333;
  background-color: #fff;
  outline: none;
}

.trigger-bootstrap-feel:focus, .trigger-bootstrap-feel:hover {
  background-color: #d4d4d4;
  border-color: #8c8c8c
}

.content-bootstrap-feel {
  background-color: #fff;
  border: 1px solid rgba(0,0,0,.15);
  border-radius: 4px;
  box-shadow: 0 6px 12px rgba(0,0,0,.175);
  padding: 5px;
  margin: 2px 0 0;
  width: 300px;
}

@keyframes drop-fade-below {
  0%   {
    opacity: 0;
    transform: translateY(-20px);
    -webkit-transform: translateY(-20px);
  }
  100% {
    opacity: 1;
    transform: translateY(0px);
    -webkit-transform: translateY(0px);
  }
}
@keyframes drop-fade-above {
  0%   {
    opacity: 0;
    transform: translateY(20px);
    -webkit-transform: translateY(20px);
  }
  100% {
    opacity: 1;
    transform: translateY(0px);
    -webkit-transform: translateY(0px);
  }
}

.slide-fade {
  will-change: transform, opacity;
}

.slide-fade.ember-basic-dropdown-content--below.ember-basic-dropdown--transitioning-in {
    animation: drop-fade-below .15s;
    -webkit-animation: drop-fade-below .15s;
  }
.slide-fade.ember-basic-dropdown-content--below.ember-basic-dropdown--transitioning-out {
    animation: drop-fade-below .15s reverse;
    -webkit-animation: drop-fade-below .15s reverse;
  }
.slide-fade.ember-basic-dropdown-content--above.ember-basic-dropdown--transitioning-in {
    animation: drop-fade-above .15s;
    -webkit-animation: drop-fade-above .15s;
  }
.slide-fade.ember-basic-dropdown-content--above.ember-basic-dropdown--transitioning-out {
    animation: drop-fade-above .15s reverse;
    -webkit-animation: drop-fade-above .15s reverse;
  }

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<div id="ember-basic-dropdown-wormhole"></div>
<br>
{{#basic-dropdown as |dd|}}
  {{#dd.trigger class="trigger-bootstrap-feel"}}NOT rendered in place{{/dd.trigger}}

  {{#dd.content class="content-bootstrap-feel width-300 slide-fade"}}
    <li>Miguel</li>
    <li>Matthew</li>
    <li>Dan</li>
  {{/dd.content}}
{{/basic-dropdown}}
<br>
<br>
{{#basic-dropdown renderInPlace=true as |dd|}}
  {{#dd.trigger class="trigger-bootstrap-feel"}}Rendered in place{{/dd.trigger}}

  {{#dd.content class="content-bootstrap-feel width-300 slide-fade"}}
    <li>Miguel</li>
    <li>Matthew</li>
    <li>Dan</li>
  {{/dd.content}}
{{/basic-dropdown}}
<p>some text below the dropdown here.</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",
    "ember-basic-dropdown": "0.33.4"
  }
}