next.js/test/development/app-dir/strict-mode-enabled-by-default/strict-mode-enabled-by-default.test.ts

import { nextTestSetup } from 'e2e-utils'
import { check } from 'next-test-utils'

describe('Strict Mode enabled by default', () => {
  const { next } = nextTestSetup({
    files: __dirname,
  })
  // TODO: modern StrictMode does not double invoke effects during hydration: https://github.com/facebook/react/pull/28951
  it.skip('should work using browser', async () => {
    const browser = await next.browser('/')
    await check(async () => {
      const text = await browser.elementByCss('p').text()
      // FIXME: Bug in React. Strict Effects no longer work in current beta.
      return text === '1' ? 'success' : `failed: ${text}`
    }, 'success')
  })
})