A per-frame landing check read 909 px below the shelf: a shoe moving straight up fooled it

// y is the flier's translateY from its computed matrix, in px: negative above the shelf line, positive below it.
// before: walk back from the end while the shoe is over its own slot
for (let i = samples.length - 1; i >= 0; i--) {
  if (Math.abs(samples[i].x) > 0.5) break;
  deepest = Math.max(deepest, samples[i].y);
}
// after: the landing is everything after the shoe was last above its shelf line
let last = -1;
for (let i = samples.length - 1; i >= 0; i--) {
  if (samples[i].y < -0.5) { last = i; break; }
}
if (last >= 0) for (const s of samples.slice(last)) deepest = Math.max(deepest, s.y);