This read only variable returns the x position (within
the room) of the left hand bounding box for the instance, where the
bounding box is defined by the maximum width and height of the mask
for the instance (as set by the sprite or by the mask index). Even
when a sprite has a precise collision mask, the bounding box exists
and is used for certain things, and so you can use this variable to
find it. Please note that when the instance has no sprite assigned
the bbox_ variables will always return 1.
if bbox_left < 0
x = sprite_xoffset;
The above code checks to see if the bounding box of the instance is outside the room and if it is it sets the instance to a new position.