New Twiddle

components.viewport-spy.js

import Ember from 'ember';
import InViewportMixin from 'ember-in-viewport';

export default Ember.Component.extend(InViewPortMixin, {
});

controllers.application.js

import Ember from 'ember';

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

styles.app.css

body {
  display: flex;
  height: 100%;
  min-height: 100%;
}

.header {
  flex: 0 0 auto;
  background: grey;
}

.header-content {
  height: 10vh;
}

.content {
  flex: 1 1 auto;
  height: 80vh;
  width: 100vw;
  overflow-y: scroll;
  background: red;
}

.footer {
  flex: 0 0 auto;
  height: 10vh;
  background: grey;
}

.scrollable {
 height: 200vh;
 position: relative;
}

.should-stick {
  position: absolute;
  top: 300px;
  height: 100px;
  width: 100%;
  background: green;
}

templates.application.hbs

<div class='header'>
  <div class="header-content">Content</div>
</div>
<div class='content'>
  {{outlet}}
</div>
<div class="footer">
  footer
</div>

templates.components.viewport-spy.hbs

{{yield}}

templates.index.hbs

<div class="sticky">
</div>
<div class="scrollable">
  <div class="should-stick">
  </div>
</div>

twiddle.json

{
  "version": "0.10.7",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.10.0",
    "ember-data": "2.10.0",
    "ember-template-compiler": "2.10.0",
    "ember-testing": "2.10.0"
  },
  "addons": {
    "ember-in-viewport": "2.1.1",
    "ember-elsewhere": "0.4.1"
  }
}