If a pre has a horizontal scrollbar it will stack thus causing the grid to malfunction.

Operating system
Windows 7 (Microsoft Windows [version 6.1.7601])

Browser and version
Chrome 61.0.3163.100

Reduced test cases
https://codepen.io/stephenbe/pen/zpXWyy

In this codepen, you can see that the first examples stack while the other without a horizontal scrollbar it works just fine.

Thanks!