Probably why you got a down vote then was because the var didn't have the onready before it and it didn't work so you show have put this:
onready var upper_left_corner = $Camera2D.global_position - OS.window_size/2
Instead of this:
var upper_left_corner = $Camera2D.global_position - OS.window_size/2
Edit: nvm for some reason it's kind of broken
Another Edit: The x is alright but the y is not