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docs: add warning for html/body tags causing infinite loop in React 19 #8380

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35 changes: 35 additions & 0 deletions src/content/blog/2024/04/25/react-19-upgrade-guide.md
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Expand Up @@ -563,6 +563,41 @@ To load React 19 with a script tag, we recommend using an ESM-based CDN such as
</script>
```

### Support for Document Metadata {/*support-for-document-metadata*/}

React 19 added support for rendering metadata tags like `<title>`, `<meta>`, and `<link>` natively in your component tree, and they will automatically be hoisted to the document `<head>`.

<Pitfall>

#### `<html>` and `<body>` tags are not supported for hoisting {/*html-and-body-not-supported*/}

While React 19 hoists `<title>`, `<meta>`, and `<link>` tags automatically, it does **not** support rendering `<html>` or `<body>` tags inside a `createRoot` component tree.

If you are migrating from `react-helmet` and render `<html>` or `<body>` directly in JSX, this will cause a **terminal infinite loop** (browser freeze) during event propagation such as input focus.
```js
// 🚩 Do not do this — causes infinite loop
function App() {
return (
<>
<html lang="en" />
<body className="dark" />
<main>...</main>
</>
);
}
```

Instead, set attributes on `<html>` and `<body>` in your HTML file or server template:
```html
<html lang="en">
<body class="dark">
<div id="root"></div>
</body>
</html>
```

</Pitfall>

### Libraries depending on React internals may block upgrades {/*libraries-depending-on-react-internals-may-block-upgrades*/}

This release includes changes to React internals that may impact libraries that ignore our pleas to not use internals like `SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED`. These changes are necessary to land improvements in React 19, and will not break libraries that follow our guidelines.
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