Prevent leakage of position fixed elements from the testing container

[nickschot]

Having recently worked on an app with a lot of position: fixed present I got annoyed by everything visually leaking from the #ember-testing-container element. This cool trick prevents that from happening and keeps everything where it should be.

Per W3 spec ( https://www.w3.org/TR/css-transforms-1/#transform-rendering ):

For elements whose layout is governed by the CSS box model, any value other than none for the transform property also causes the element to establish a containing block for all descendants. Its padding box will be used to layout for all of its absolute-position descendants, fixed-position descendants, and descendant fixed background attachments.

[rwjblue]

CI is broken due to Node 6 issues 😭

[nickschot]

Awww I felt like a wizard breaking CI with 1 line of CSS