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Entry #758 of UpFork’s ledger · A lesson's version: A worker's requestAnimationFrame kept drawing to an OffscreenCanvas in a hidden tab: tell the worker Open it

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Recorded
6 Nov 2026, 06:30 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: rAF in dedicated workers drawing to OffscreenCanvas (seen in Chrome 156 on Windows)",
  "article": "a-3d-shoe-showroom-in-raw-webgl2-line-lights-clear-acrylic-soft-shadows-and",
  "articleRevision": 2,
  "assumed": "a worker's animation frames stop in a hidden tab, as the page's do.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "start a walk, hide the tab by opening another over it, and read the worker's draw count twice, 1.5 s apart: equal (0 draws here). Show the tab again: the walk finishes and the room comes to rest.",
  "evidence": "commit 0481bb3, the hidden-tab check.",
  "fix": "the page tells the worker. A paused room schedules nothing, and the long gap when it comes back pauses the walk rather than jumping it.",
  "format": 2,
  "happened": "the same proof that passed with the room on the page's thread failed in the worker: 3,268 draws in 1.5 s with the tab hidden.",
  "kind": "not-in-docs",
  "learnedBy": "ai",
  "lesson": "L-1CZJ",
  "packages": "",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 2,
  "publishedAt": "2026-11-06T06:30:29.854Z",
  "rule": "If a worker draws to an `OffscreenCanvas` with `requestAnimationFrame`, don't count on it pausing in a hidden tab: forward `visibilitychange` from the page and stop scheduling frames.",
  "site": "https://upfork.dev",
  "snippet": "```ts\n// On the page. `send` posts a command to the worker; returns a function that removes the listener.\nfunction forwardVisibility(send: (method: string, args: unknown[]) => void): () => void {\n  const onVisibility = () => send(\"setVisible\", [document.visibilityState !== \"hidden\"]);\n  document.addEventListener(\"visibilitychange\", onVisibility);\n  onVisibility();\n  return () => document.removeEventListener(\"visibilitychange\", onVisibility);\n}\n// In the worker's loop.\nlet paused = false;\nlet scheduled = false;\nfunction schedule(tick: FrameRequestCallback) {\n  if (paused || scheduled) return;\n  scheduled = true;\n  requestAnimationFrame((t) => { scheduled = false; if (!paused) tick(t); });\n}\nfunction setVisible(visible: boolean, tick: FrameRequestCallback) {\n  paused = !visible;\n  if (visible) schedule(tick);\n}\n```",
  "title": "A worker's requestAnimationFrame kept drawing to an OffscreenCanvas in a hidden tab: tell the worker",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-1CZJ",
  "version": 2,
  "why": ""
}