Plank appears to be drawing a shadowed line across the lower eighth of the screen, well above plank itself. This line appears to be sticky - windows will snap to it. It disappears when the plank process is killed, and when plank is set to appear on the right instead of the bottom, the shadowed line becomes a vertical line to the left of the dock. It doesn't seem to harm anything, but it's kind of an eyesore - A quasi-bifocal line for my screen! I am using plank with xfce 4.12, and the nvidia graphics driver. This appears to be something that started recently.
(In reply to ucsdboy from comment #0) It's not bug, it's new feature introduced since Xfce 4.12, see my post [1] on FreeBSD forum, in order to "fix" this behaviour. [1] https://forums.freebsd.org/threads/desktop-issue-after-upgrade.51207/#post-287133