UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.101 Safari/537.36

Steps to reproduce the problem:
1. Create a document with nested elements, all of which have "position: fixed". 

2. Style each nested element such that they cover different portions of the viewport, e.g. four nested <div>s which are styled "width: 33%, height: 33%;" and positioned in the four corners of the viewport: "left: 0; top: 0;", "left: 0; bottom: 0;", "right: 0; top: 0;", "right: 0; bottom: 0;"

3. Apply any transform (e.g., "transform: rotate(0deg)") one or more outer nested elements.

4. Observe that the inner element adjusts its dimensions as if the inner element were its containing context, instead of the viewport

What is the expected behavior?
Fixed position elements should always be sized and positioned relative to the viewport. 

What went wrong?
Applying any transform to the outer fixed-position element causes nested fixed-position elements to be incorrectly laid out.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 60.0.3112.101  Channel: stable
OS Version: OS X 10.12.5
Flash Version: 

Demonstration:
https://codepen.io/anon/pen/mMXjjE?editors=1100

Uncomment one or more of the transform rules in the CSS.