Hi, it's me again.
I noticed that the vertical scroll introduces a "ghost line." I thought I was doing something wrong, but then I tried one of the examples from your tool and the same thing happens, and I can't delete it.
Hello, again!
Yes, the playfield does have an extra row that only becomes visible after scrolling:
https://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#pfscroll
You can even draw to it using the pfpixel command.
Clever use of this trick can be used to implement infinite scrolling.