[Eugene (OEvgeny)]

Related to #5552 and #5850: the IIFE build of the Fluent theme crashes on load when the page uses React 19.

Changelog Entry

Fixed

• Fixed the IIFE build of the Fluent theme crashing on load with React 19, in PR #5971, by @OEvgeny

Description

The Fluent theme IIFE bundled React DOM 16.8.6 (via renderWebChat(), which it does not export) and evaluated it against window.React, which throws on React 19; it now uses window.ReactDOM, like window.React. Also folds in #5963's review comments, a docs fix, and React 19 tests.

Design

• Map react-dom to the page's ReactDOM in the IIFE tsup plugin, like react; tree-shaking cannot drop it, the bundled module runs on load.

Specific Changes

• renderWebChat.tsx: split the render schema into legacyReactDOMSchema and reactRootSchema, name ReactDOM.createRoot() in the error
• fluent-theme/tsup.config.ts, src/external.umd/react-dom.ts: IIFE builds resolve react-dom to window.ReactDOM
• react19/fluentTheme.iife.html: snapshot 1, Fluent theme on React 19 shows a sent message and the reply
• docs/HOSTING.md: remove react-dom/client from the baseline import map, it is never built
• renderWebChat.spec.tsx: an error from root.render() is thrown synchronously
• react19/codeBlockCopyButton.html: snapshot 1, the copy button shows "Copied" on React 19
• react19/*.html: expect ReactDOM.version; the React 18 root test spies with jest-mock

• I have added tests and executed them locally
• I have added .changelog/<PR#>-<slug>.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

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