there seems to be an off-by-1 error once you start scrolling the screen and it goes past what I assume is a page boundary.
I suspect it's probably a simple maths bug when wrapping or something similar, but I imagine it has the potential to be causing less obvious bugs elsewhere too.
It's always been like this in MAME, but real hardware videos don't seem to show the glitch, so it might be a good one to look at.
