One of the basic problems here is the semantics of "scroll". If you have a full screen of text and then "add a line at the bottom" then you're going to scroll everything up by one line. That's pretty clear.
Similarly, if you have a text editor that divides the screen into two horizontally-tiled buffers, you can define the scroll region (top margin/bottom margin) appropriately and have the same semantics.
But if you have vertically-tiled buffers (open Emacs, C-x 3) then the top margin/bottom margin model no longer works. If you "scroll" one of those buffers, you are doing it to both of them, and you need to overpaint the other one afterwards; on a fast enough connection that's invisible within standard terminal semantics but will have odd visual artifacts for a smooth scroll.
Similarly, if you have a text editor that divides the screen into two horizontally-tiled buffers, you can define the scroll region (top margin/bottom margin) appropriately and have the same semantics.
But if you have vertically-tiled buffers (open Emacs, C-x 3) then the top margin/bottom margin model no longer works. If you "scroll" one of those buffers, you are doing it to both of them, and you need to overpaint the other one afterwards; on a fast enough connection that's invisible within standard terminal semantics but will have odd visual artifacts for a smooth scroll.