When it gets to the last full line and I hit Down one more time, the file does indeed scroll up, but by two lines instead of one.
As you may have noticed, Zeus by design always keeps the cursor on the screen. It does this by checking the cursor every time it moved.
So what I think you are seeing is the scroll down action hitting the bottom of the page resulting in first line scroll, then the cursor checking code finding part of the cursor is off the screen, resulting in the second line of scroll.
The difference you are seeing between the two version is recently the cursor checking code was change to make sure the entire cursor is on the screen.