Object indices, ended by $FF, of every object that iy_becomes_candidate has made the candidate since render_obj last drew something. iy_goes_first looks for an object here before making it the candidate: finding it means the depth order has a cycle. The first byte is set to $FF after every draw, so between frames the list is always empty. The object the pass began with is not recorded, so a cycle back to it is caught one step later, at the next object in the loop.