Originally Posted by Haze
is that method of scrolling with a bitmap layer known to be used anywhere else?

It is, and what do you know, the split shows up there too. So it's not game-specific:

[Linked Image from i.imgur.com]

I've checked it out in UNZ and the VRAM has the same content, but the part after the split looks different in movement, like it "shakes" a bit.