I have this problem on a macbook pro with retina screen and OSX 10.9.2
Total Finder 1.5.38
external screen is 1920x1080
set laptop screen to scaled resolution 1920x1200
have laptop screen as primary (with dock).
visor set to appear on primary (laptop screen)
dock set to autohide
When I toggle the visor, sometimes it leaves some garbage at the bottom of the screen.
Seems to be around 120 pixels high.
This garbage flickers when the dock animates in and out…
If I set my internal laptop screen to scaled resolution 1680x1050 the problem disappears.