Flexbox wonky

controllers.application.js

import Ember from 'ember';

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

styles.app.css

* {
  box-sizing: border-box;
}

html {
  height: 100%;
  min-height: 100%;
  margin: 0;
}

body {
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
  height: 100%;
  min-height: 100%;
  margin: 0;
  background-color: rgba(0, 0, 0, .05);
  padding: 16px
}

#root.ember-application {
  position: absolute;
  height: 70%;
  width: calc(100% - 20px);
}

#root.ember-application > div.ember-view {
  display: flex;
  flex-direction: row;
  flex: 1 0 auto;
  height: 100%;
  background: green;
  padding: 8px;
}

.callscreen-main {
  padding: 8px;
  display: flex;
  flex-direction: column;
  background: red;
  border: 1px solid;
  height: 100%;
}

.header-bar {
  padding: 8px;
  flex: 0 0 auto;
  border: 1px solid;
  margin-bottom: 8px;
}

.static-content {
  padding: 8px;
  flex: 0 0 auto;
  border: 1px solid;
  margin-bottom: 8px;
}

.scrollable {
  height: 100%;
  flex: none;
  position: relative;
}

.flex-content {
  position: relative;
  padding: 8px;
  flex: 1 1 auto;
  border: 1px solid;
  overflow-y: scroll;
}

templates.application.hbs

<div class="callscreen-main">
  <div class="header-bar">
    Welcome to Flexbox
  </div>
  <div class="static-content">
    <p>Static Content</p>
    <p>Static Content</p>
    <p>Static Content</p>
    <p>Static Content</p>
    <p>Static Content</p>
  </div>
  <div class="flex-content">
    <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
      <p>Flex/Scroll Content</p>
  </div>
</div>

twiddle.json

{
  "version": "0.9.3",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-data": "2.6.1",
    "ember-template-compiler": "2.6.0"
  }
}