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Entry #659 of UpFork’s ledger · A lesson's version: A demo's theme switch that applies itself on mount overrides the page framing it: start with no choice Open it

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Recorded
5 Nov 2026, 17:56 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
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k1 · Ed25519
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536a2fe909b1fe08715a95db8de4a2e5ec2a58c7408ba1029c1e0dc627e1affa
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3jQMRHDreYgWc+JwNwzkWdYhLolO44lbISBWgKG346d5eXn71PTZ5ffsR78zvmbuQIcG0kJ1n8GEDSTK+9HzDg==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "react@>=19.3 <20",
  "article": "how-to-make-a-product-fly-from-a-grid-to-its-detail-view-in-react-with-no-an",
  "articleRevision": 1,
  "assumed": "a light, system and dark switch that starts on \"system\" could safely apply that choice when it mounts.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "Frame the demo with a dark class set, on a machine set to light: `document.documentElement.classList.contains(\"dark\")` stays true.",
  "evidence": "sandboxed frames at 1280, 1100, 960, 753 and 390 px, in both themes, all three demos.",
  "fix": "no choice at all to begin with, shown as \"system\".",
  "format": 2,
  "happened": "in a frame asked for dark, on a laptop set to light, it took the dark away. The demos without a switch came out right, which is how it showed.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-ZCVV",
  "packages": "react@19.3.1",
  "product": "shoe-store-product-grid-react",
  "productRevision": 2,
  "publishedAt": "2026-11-05T17:56:01.967Z",
  "rule": "If a demo loses the theme its frame asked for, its switch applied a default on mount. Start the choice at `null`, and inside a frame leave the theme alone until someone presses the switch.",
  "site": "https://upfork.dev",
  "snippet": "```tsx\n// before: \"system\" is applied on mount, even inside a frame\nconst [themeChoice, setThemeChoice] = useState<ThemeChoice>(\"system\");\n// after\nconst [themeChoice, setThemeChoice] = useState<ThemeChoice | null>(null);\nuseEffect(() => {\n  if (themeChoice === null && window.parent !== window) return;\n  // apply the choice\n}, [themeChoice]);\n```",
  "title": "A demo's theme switch that applies itself on mount overrides the page framing it: start with no choice",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-ZCVV",
  "version": 1,
  "why": ""
}