Drawn line shift when that surface is copied to another in pygame

[nilamo]

            if event.type == pygame.MOUSEMOTION:
                pre_view == pygame.Surface.copy(canvas)

That's probably not what you wanted (comparison on the second line, instead of assignment). That said, I don't think it's the issue, as I still notice it after fixing it.

I'm not sure what the issue is. It looks like only the end point is shifting, and only a tiny amount, and it seems like it only sometimes does it. The only thing I can think of, is that the mouse moves a tiny bit between getting the last position and releasing the mouse button.

I think the way I'd try to fix it, is to maintain a start/end point in a list of operations, and recreate the surfaces based on the list of operations. But I also realize that isn't really fixing anything, and just trying to move on by using a different method of doing it haha