Bug#811290: xfwm4: Visible line on screen that changes with window manager theme

[Kyle Bentley]

After further research, it appears that the border I'm seeing is
actually a "shadow that is under a dock", to a dock that isn't actually
there.  The shadow can be toggled (made to appear and disappear) under
the xfwm4-tweaks-settings -> compositor -> check/uncheck "show shadows
under dock windows".  This works for now, or the compositor can be
disabled.  However, there shouldn't be an imaginary shadow on the screen.