The mosaic effect is a horizontal "pixelation" (sample-and-hold) filter.
The sampling interval can be set from 1 to 16 pixels, and it can be enabled for each layer (except pivot it's strange that this is the case, but i suspect the "pivot port" system can be used instead).
The columns at which the sampling occurs are fixedshow the math to determine the sampling positions., and the effect is reset 2 pixels before the right edge of the screen also we should check: at the right edge, which line's mosaic parameters are used?
Lineram Register 2.2 "6400"
#:8,mosaic:4,#:4
mosaic
Mosaic: Pixelation Amount samples every [16−mosaic] pixels