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Recorded
14 Nov 2026, 09:05 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
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k1 · Ed25519
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{
  "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": ""
}