Board Thread:Support Requests - Designing Your Wikia/@comment-3147293-20141104031847/@comment-452-20141104195832

The border is cut off because you've got it set as 100% (300px), with a 1px border = 302px.

The use of "box-sizing: border-box;" will sidestep that problem and show the border.

edit: However, this should have been a problem even at 330px + 1px border = 332px.

this archive from 20130121 shows that it was.