I don't know why this was so hard, but I don't need this variable preserve_screen_top_of_cursor_node at all. We only set it when the cursor is in some node, but we also only check for when the current node is the cursor. Comparing with a nil cursor node works just as well.

I've also checked that driver.love doesn't need preserve_screen_top_of_cursor_node. I think it came from pensieve.love, where I've since taken it out. Did I ever need it even there?