Occurs when using the setSlot call to move a control.
If your electra is configured that Active Control Set is set to Background and you then use a Lua script to move a control from one of the none active areas on the screen into the Active control set the background of the control will not be painted with the same background colour as the rest of the controls in the Active Control Set.
This is on Electra MK2 3.6.1 firmware