Because of the
width: 755px, the small-action can cause a horizontal scroll bar (even on meta) when you resize the window.
The small-action class is used for the time gap, closed notifications, et al.
I’ve submitted a PR that solves this issue by using
max-width: 755px which ensures when there are 755 or more pixels available, the border will carry the same length as a post, and when there is less, it still takes up the same width as a post.
max-width when there is more than 755px available
when there is less than 755px available
when there is less and not using max-width (without the PR)