Out of place lines are being printed to the screen in performance mode. The lines only blit onto the screen for a frame or two and disappear immediately after that. They'll show up when you mouse over the side chains or while moving. The extraneous bar tends to stay in the same place for a while and after several moves it'll appear in a new place on the screen. It's almost like the video buffer used to initialize the display hasn't been fully cleared out... a "one-off" error. It's not a serious problem

I am seeing this in the Mac-beta version.