André Nicolas wrote an answer to this question, in which he mentions "the line through $A$ and $B$." It appears in my browser window as:
the line through $A$ and $B$
with the period on the next line. Maybe that changes with the window geometry, or maybe not. But that shouldn't happen.
(Usually when I see this it's because someone typed some "displayed" TeX in which they put the period OUTSIDE of TeX, and the fix is to move it inside, where it should have been. But that doesn't apply here.)
Later note: In this posting, the period sits alone on the next line after displayed TeX, simply because the line is so long that it starts wrapping right at the point where the only thing left to display is the period!
A screen shot of a similar problem provided in a duplicate report: