This read only variable returns the y position (within
the room) of the bottom of the 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
if bbox_bottom > room_height
y = room_height - sprite_yoffset;
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.