When an icon doesn't define the "" state and you set an atom's icon_state to "", it forgets how wide or tall it is. For example, if you have a 192x192 graphic, and your pixel_x is -64, and you set the icon_state equal to "" but there's no "" state defined in the icon, mouse_opacity will be shifted 64 pixels to the west instead of working on the actual loc of the atom...

This is tested with tile-based game and a default bound_width equal to world.icon_size