Bug: AlignWide content clipped

At some viewport sizes, content with the `alignwide` class is being clipped by the viewport. In the attached screenshot, the viewport is 1265px wide, but the `alignwide` content is 1280px wide. I'm guessing the `vw` calculations are off somehow — perhaps due to visible scrollbars?

Reproduce the clipping at the reported 1265px viewport and inspect the styles for the `alignwide` class, focusing on the `vw` calculations and scrollbar width. Done means alignwide content remains within the viewport at the affected sizes.

Written by the indexing model from the issue text.