.py

# Being called when loaded.
func _ready():
    screen_size = self.get_viewport_rect().size

# Actually scroll the screen. (Update the viewport according to the position of the camera.)
func update_viewport():
    var canvas_transform = self.get_viewport().get_canvas_transform()
    canvas_transform.o = -self.get_global_pos() + screen_size / 2.0
    self.get_viewport().set_canvas_transform(canvas_transform)