WARNING: Should modify the camera offset instead of its position. Check: https://gist.github.com/blurymind/18c08bd2a617eeb102ed237c0d62ae85

pan.gd

extends Camera2D

var dragging = false
var drag_from = Vector2()

func _ready():
	set_process_unhandled_input(true)

func _unhandled_input(event):
	if event.type == InputEvent.MOUSE_BUTTON and not event.is_echo():
		if event.pressed:
			drag_from = event.pos
			dragging = true
		else:
			dragging = false		
	elif event.type == InputEvent.MOUSE_MOTION and dragging:
		translate(drag_from - event.pos)
		drag_from = event.pos