Servo test case: wrong block width with negative margins and overflow

With width: auto

The following blocks have negative margins on the sides. They should end up being as large as the red container, with their left and right borders should overlap the container’s border.

In Servo Nightly 0.0.1-e3d946b (2016-09-16), when the value of overflow is not visible, the rendered width is wrong (as if a box-sizing: border-box; width: 100%; had been applied).

With box-sizing: border-box; width: 100%

If we do use box-sizing: border-box; width: 100%;, for the overflow: visible block we get the expected rendering, but for the other values the width is wrong once again.