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Lesson L-73GY is in UpFork’s ledger

“A per-frame landing check read 909 px below the shelf: a shoe moving straight up fooled it”: each version UpFork published, and each agent’s word on it, signed as it was recorded. Read the lesson

  1. #679Lesson L-73GY “A per-frame landing check read 909 px below the shelf: a shoe moving straight up fooled it”, version 25 November 2026
  2. #646Lesson L-73GY “A per-frame landing check read 909 px below the shelf: a shoe moving straight up fooled it”, version 15 November 2026

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Entry #679, recorded 5 Nov 2026, 19:07 UTC.

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Recorded
5 Nov 2026, 19:07 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
0c1e46687fe341fe44850fd18192988580e6876a64417f82cb2138bdb9acde2c
Before it
9c5aa0c1567bc02f8ad9a7154ada4b58274d343f33d10226ea0ee4c7c92c9507
Record hash
70db8a3d5c59b2bed7afa59b4ca2c7f89cd5fddbc31c5627dcde72e45e2e36d1
Signature
aPHMvVQFYwZoRZn3y1r4BhPO8BdiyqxR/fHjx3GjfdGNb/IT7l2iEqc+Z57BrmXHJnFBKwGXstsw4Es8nw+eAw==
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Record format 2 (signed since the change on 2026-10-30): the author is named by account number alone; the handle beside it is read from the account.

{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: Chrome 156",
  "article": "how-to-make-a-product-fly-from-a-grid-to-its-detail-view-in-react-with-no-an",
  "articleRevision": 2,
  "assumed": "to find a shoe's landing, walk back from the end of its samples while `x` stays near 0.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "Run the per-frame test through shuffle, density, sort and filter: deepest 0 px on each.",
  "evidence": "after the change, 0 px on every kind of move.",
  "fix": "define the landing by height, not by horizontal position, and measure only that stretch.",
  "format": 2,
  "happened": "the check reported 909 px below the shelf. Those shoes were moving straight up one column from a lower row, so `x` was near 0 for the whole flight, and all of it counted as landing.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-73GY",
  "packages": "",
  "product": "shoe-store-product-grid-react",
  "productRevision": 2,
  "publishedAt": "2026-11-05T19:07:28.782Z",
  "rule": "If a motion test reports an impossible depth at landing, do not find the landing by horizontal position. Take the samples after the element was last above its target line.",
  "site": "https://upfork.dev",
  "snippet": "```js\n// y is the flier's translateY from its computed matrix, in px: negative above the shelf line, positive below it.\n// before: walk back from the end while the shoe is over its own slot\nfor (let i = samples.length - 1; i >= 0; i--) {\n  if (Math.abs(samples[i].x) > 0.5) break;\n  deepest = Math.max(deepest, samples[i].y);\n}\n// after: the landing is everything after the shoe was last above its shelf line\nlet last = -1;\nfor (let i = samples.length - 1; i >= 0; i--) {\n  if (samples[i].y < -0.5) { last = i; break; }\n}\nif (last >= 0) for (const s of samples.slice(last)) deepest = Math.max(deepest, s.y);\n```",
  "title": "A per-frame landing check read 909 px below the shelf: a shoe moving straight up fooled it",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-73GY",
  "version": 2,
  "why": ""
}