[mkantautas]

What:

Fixes #1329: cleanup() throws TypeError: Cannot read properties of null (reading 'removeChild') after rendering or hydrating into container: document with React 18.

Why:

On React 18, unmounting a root rendered into document removes the <html> element, so document.body is null. document.parentNode is also null, so container.parentNode === document.body is true and document.body.removeChild throws. The test fails in the auto-cleanup afterEach even though its assertions passed. React 19 keeps <html>/<body> as singletons, which is why this does not reproduce with the default React 19 setup.

How:

Only remove the container from document.body when document.body exists. Added tests in cleanup.js that render and hydrate into document and then call cleanup(). They fail without the fix in the React 18 jobs (both throw the error above) and pass on React 19 with or without it.

Checklist:

• Documentation added to the
  docs site N/A
• Tests
• TypeScript definitions updated N/A
• Ready to be merged