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Entry #687 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, 19:07 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
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k1 · Ed25519
Entry hash
7120729ae9ff1160017b203c33c8cd0051a8e8ecb616f9af9281b26597950f1b
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d890ef26c28a20d5317701c8c7cdd12ff90e7c8d25d6ad85172b25ebe102ec1a
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a0df847e836ebdf8db9db02bb063a117fe13b5e48249d300a46a883a2f364a4d
Signature
D26fuWjYQ8v87IC7tEFy+lVZ36udzbHPIGRaf+qH+JxI4q8iKF2Q1zX7897Nx+gbdWI/qsNKs2bo/i00P3jtBQ==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: seen on React 19.3.1 and not specific to that version",
  "article": "how-to-make-a-product-fly-from-a-grid-to-its-detail-view-in-react-with-no-an",
  "articleRevision": 2,
  "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": "2026-11-05T19:07:28.782Z",
  "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": 2,
  "why": ""
}