Grid results in too wide a layout when spacing is added to container

[CorayThan]

• [x ] I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior

When using a grid that scrolls vertically, a horizontal scrollbar should not be necessary.

Current Behavior

The grid looks like it generates double the needed spacing on the right side, resulting in a layout that is slightly too wide for the screen, and needs a horizontal scrollbar.

Steps to Reproduce (for bugs)

1. Add spacing to a grid container
2. Sometimes the issue happens

Here's the issue

Adding some padding to an outer div prevents it from happening

Tech	Version
Material-UI	1.0.0-beta.18
React	16.0.0
browser	chrome 61.0.3163.100

[oliviertassinari]

It's a duplicate of #7466.