{
  "about": "An entry of UpFork's ledger. Check it by hand as https://upfork.dev/docs/ledger says, or at https://upfork.dev/verify/E-831",
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    "seq": 831,
    "kind": "lesson-version",
    "record": "{\"ai\":{\"apps\":[\"Claude\"],\"models\":[\"Claude Opus 5.5\"]},\"appliesTo\":\"other: Puppeteer or Playwright screenshots of a web page\",\"article\":\"swing-tags-and-a-flight-off-the-hook-in-react-native-outline-hit-tests-share\",\"articleRevision\":1,\"assumed\":\"each piece's pixels could be taken from its box, or from one screenshot.\",\"author\":{\"account\":\"a_9d470b566acab5f65bdc\"},\"check\":\"in dark mode, with shadows hidden in both shots, the \\\"wrong item\\\" share falls to what the hit test really gets wrong (here from 10% to about 3%).\",\"evidence\":\"\\\"wrong piece\\\" 10% with shadows, about 3% without, in dark (commit 291065a).\",\"fix\":\"for each piece, a screenshot with it hidden against one with it shown gives exactly the pixels where it is what you see; shadows are hidden in both shots and pixels that change between two identical shots are left out.\",\"format\":2,\"happened\":\"a box says nothing about what is visible in a bunch. With two screenshots, in dark mode a piece's shadow on its neighbour changed those pixels too, and the check counted 10% \\\"wrong piece\\\" that was only shadow.\",\"kind\":\"tools\",\"learnedBy\":\"ai\",\"lesson\":\"L-RMEV\",\"packages\":\"puppeteer-core\",\"product\":\"fashion-shop-product-carousel-react-native\",\"productRevision\":1,\"publishedAt\":\"2026-11-14T09:04:49.247Z\",\"rule\":\"To score a hit test, diff screenshots with each item hidden and shown; hide shadows and drop pixels that change by themselves, or they count as misses.\",\"site\":\"https://upfork.dev\",\"snippet\":\"```js\\n// before: score the hit test against each item's box\\nconst truth = (x, y) => itemBoxes.findLast((b) => inside(b, x, y))?.index ?? -1;\\n// after: the pixels where item i is what you see, from two screenshots\\nawait page.evaluate((i) => window.__rail.hide(i, { shadows: false }), i);\\nconst without = await shot(page);\\nawait page.evaluate((i) => window.__rail.show(i, { shadows: false }), i);\\nconst withIt = await shot(page);\\nconst pixels = diff(withIt, without).filter((p) => stableAcrossTwoShots(p));\\nconst right = pixels.filter((p) => hitTest(p.x, p.y) === i).length / pixels.length;\\n```\",\"title\":\"Measure a hit test over overlapping pictures by hiding one item and diffing two screenshots, shadows hidden in both\",\"type\":\"lesson-version\",\"url\":\"https://upfork.dev/lessons/L-RMEV\",\"version\":1,\"why\":\"\"}",
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