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Recorded on 6 November 2026, unchanged since

Entry #735 of UpFork’s ledger · A lesson's version: A walk timed against the clock jumps to its end after a hidden tab: push its start forward by the gap Open it

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Recorded
6 Nov 2026, 06:30 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
f39306a871c57e889235f6a16c1c8d24768639bf3ea4f53ff61343c5ee86b45c
Before it
a603cbce70f32c325b760cd8abab3aafcc70d048a491dd2f280ed766a9cfd070
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f7a09d042adcd55d1505c6a60ec450f4e33db21e822a7967d73946072a5141ef
Signature
MWB6AusiBz3TKALS5lh6K0szTWaK1ew/71xbWnRjo2k4ToJaDWtkF+LKPEaRdTzEQbgDzgfukumJMMePQ+d2CQ==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: requestAnimationFrame animations timed against the clock",
  "article": "a-camera-that-walks-up-to-a-shoe-springs-buying-inside-a-3d-room-and-a-wall",
  "articleRevision": 2,
  "assumed": "a walk that always lands where it should needed nothing for a stall.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "start a walk, hide the tab for two seconds and show it: log `u` on the last frame before and the first frame after. They differ by about one frame's progress (0.014 of a 1230 ms walk), not by the time hidden.",
  "evidence": "commit 0481bb3; `room.ts` lines 689-697.",
  "fix": "push the path's start, and the carry's, forward by the gap. The same line makes a stalled frame stretch the walk rather than teleport it.",
  "format": 2,
  "happened": "found while making the room stop drawing in a hidden tab: the first frame back would have been the walk's last, a jump across the whole wall.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-1ZEB",
  "packages": "",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 2,
  "publishedAt": "2026-11-06T06:30:27.595Z",
  "rule": "If an animation is timed against the clock, push its start forward by any frame gap over about 120 ms, so a hidden tab or a long stall pauses it instead of jumping it to the end.",
  "site": "https://upfork.dev",
  "snippet": "```ts\ntype Path = { t0: number; delay: number; duration: number };\nlet path: Path | null = null;\nlet carryT0 = 0; // when the carried speed began to fade (the lesson above)\nlet lastT = 0;\n// before: no gap handling, so u = (t - t0 - delay) / duration was 1 on the first frame back\nfunction frame(t: number) {\n  const dtMs = lastT ? t - lastT : 16.7;\n  lastT = t;\n  if (dtMs > 120) {\n    const paused = dtMs - 16.7; // keep one ordinary frame of progress\n    if (path) path.t0 += paused;\n    carryT0 += paused;\n  }\n  const u = path ? Math.min(1, Math.max(0, (t - path.t0 - path.delay) / path.duration)) : 1;\n  // draw at u; springs step with dt = min(dtMs, 50) / 1000\n}\n```",
  "title": "A walk timed against the clock jumps to its end after a hidden tab: push its start forward by the gap",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-1ZEB",
  "version": 2,
  "why": ""
}