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Recorded on 6 November 2026, unchanged since

Entry #748 of UpFork’s ledger · A lesson's version: A swipe timed in its handlers is only as quick as a busy thread: time it with the events' own timeStamp Open it

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Recorded
6 Nov 2026, 06:30 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
a07129e35fa7f3ce672735adec5803bb5c7a038519b0fe60b402b612e689f828
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dba0a233532f0989fb1e079c0b98a371c444565653a6957f796d45a1dd29447c
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cbc3c2edb0848147c18234971f734239ed429796bf4741f7b92ad626814b6283
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3oMehgWaZ+E97kIgcfok9osz1TtT/iDJSaepV0kU0CvvgZ74CkBBqNS492KI3jU0W1ELZ9wCoeYlt6u4r/wLAg==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: touch gestures with a time limit, other: tests that drive touch over CDP",
  "article": "a-camera-that-walks-up-to-a-shoe-springs-buying-inside-a-3d-room-and-a-wall",
  "articleRevision": 2,
  "assumed": "a handler runs when the finger moves, so its clock is the finger's.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "dispatch a 0.2 s swipe with timestamps while the page is busy: the walk starts (on all seven sizes here).",
  "evidence": "commit 6fc7667; `scripts/qa/motion-sweep.mjs`.",
  "fix": "`timeStamp` in the component, and stamped events in the test.",
  "format": 2,
  "happened": "in the sweep, swipes on the larger tablets did nothing. By the events' own clocks, each 0.2 s drag had lasted 1.0 to 1.8 s: CDP stamps an event when it arrives, and the busy page took each one slowly. On a real phone the finger's timing is in `timeStamp`, but the handlers read `performance.now()`, which a busy thread stretches too.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-MSWM",
  "packages": "",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 2,
  "publishedAt": "2026-11-06T06:30:27.595Z",
  "rule": "If a gesture has a time limit, measure it with `event.timeStamp`, not `performance.now()` in the handler; in a CDP test, give each touch event a timestamp, or a busy page stretches the swipe.",
  "site": "https://upfork.dev",
  "snippet": "```tsx\n// walk(forward) and close() are the component's own actions.\nconst touchStart = useRef<{ x: number; y: number; t: number } | null>(null);\nconst onTouchStart = (e: React.TouchEvent) => {\n  const t = e.touches[0];\n  touchStart.current = { x: t.clientX, y: t.clientY, t: e.timeStamp }; // before: t: performance.now()\n};\nconst onTouchEnd = (e: React.TouchEvent) => {\n  const from = touchStart.current;\n  touchStart.current = null;\n  if (!from || e.timeStamp - from.t > 700) return; // before: performance.now() - from.t\n  const t = e.changedTouches[0];\n  const dx = t.clientX - from.x, dy = t.clientY - from.y;\n  if (Math.abs(dx) > 48 && Math.abs(dx) > Math.abs(dy) * 1.4) walk(dx < 0);\n  else if (dy > 70 && dy > Math.abs(dx) * 1.4) close();\n};\n```\n```js\n// A test over CDP: each event stamped as a finger would make it.\nconst t0 = Date.now() / 1000;\nawait cdp.send(\"Input.dispatchTouchEvent\", { type: \"touchStart\", touchPoints: [{ x: 300, y: 300 }], timestamp: t0 });\nawait cdp.send(\"Input.dispatchTouchEvent\", { type: \"touchMove\", touchPoints: [{ x: 100, y: 300 }], timestamp: t0 + 0.2 });\nawait cdp.send(\"Input.dispatchTouchEvent\", { type: \"touchEnd\", touchPoints: [], timestamp: t0 + 0.216 });\n```",
  "title": "A swipe timed in its handlers is only as quick as a busy thread: time it with the events' own timeStamp",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-MSWM",
  "version": 1,
  "why": ""
}