The line is in place of where the scrollbar would be. Here is a picture of what I mean:
And here is the CodePen:
I don’t believe I’ve done anything odd to cause this.
Based on the colour of the line (#F1F1F1 in Chrome, and #F8F8F8 in Firefox) it appears to be the remains of the otherwise removed scrollbar.
Not a big problem, but I assume not intended behaviour!