Upon research, finding, and testing this bug, it appears that the latest tip of Bespin already had this issue fixed. See comments below which are found on the issue page in bugzilla:

Patrick Walton 2010-01-27 11:19:04 PST

Adding new lines to the buffer causes a one pixel wide line in the color of the

selection to appear on the left side of the editor view.

[reply] [-] Comment 1 Patrick Walton 2010-02-19 19:14:55 PST