UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.186 Safari/537.36

Steps to reproduce the problem:
Reduced Test Case Here
https://codepen.io/micahgodbolt/pen/LQXovj

What is the expected behavior?
black div should line up with grey divs

What went wrong?
black div is offset several pixels

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 64.0.3282.186  Channel: stable
OS Version: 10.0
Flash Version: