#geometry

On a track that crosses itself (a figure-eight, or any lap that revisits the same ground), a per-frame nearest-waypoint lookup jumps about half a lap at the crossing, because two points that sit close together can be arclength-distant. Drive lap position from a monotone cursor that only advances along the committed branch, do lateral math in a left-of-travel frame, and keep the branch-discrimination cone tighter than the branches' angular separation.