I have scrolling enabled on top edge and just added a second monitor to my workstation. In the Gnome display settings I was trying to move the second display a tiny bit up relative to my main display to mirror the real-world relative height. However, this causes the "top edge" to only exist on the higher display, which in this case is my secondary. I would prefer to be able to scroll on both displays but really need it to work on my primary display, regardless of relative positioning of the displays. It should be possible to use the bottom edge of my main display as a weak substitute.
Even worse, if I want to use my secondary display in portrait mode then, since it's taller than my main display, it voids both the top and bottom edges. This is likely to be my main use-case for the second monitor so this is a big problem for me.
EDIT:
I see that you're calculating the top-most and bottom-most edges based on the relative positions of the displays. I see why this is necessary since you can't necessarily predict what the configuration of the displays is and what edges are actually borders between monitors.
To be clear, my displays are side-by-side so I don't expect to be able to scroll on the inner edge of either display. If I wanted to use the left or right edges, I would only expect to be able to scroll on the outer edges of my combined workspace. Maybe the extension can have a user-configurable setting to set which edges of which display should be scrollable regardless of relative position.