bug: calling configure() without reactStrictMode resets it to undefined, silently disabling strict mode #1466

function configure(newConfig) {
  if (typeof newConfig === 'function') {
    newConfig = newConfig(getConfig())
  }

  const {reactStrictMode, ...configForDTL} = newConfig

  configureDTL(configForDTL)

  configForRTL = {
    ...configForRTL,
    reactStrictMode,          // <-- always overwrites, even when undefined
  }
}

import { configure, getConfig } from '@testing-library/react'

// Enable strict mode
configure({ reactStrictMode: true })
console.log(getConfig().reactStrictMode)  // true ✓

// Later, an unrelated configure() call — e.g. from a setup file
configure({ asyncUtilTimeout: 2000 })
console.log(getConfig().reactStrictMode)  // undefined ✗  (expected: true)

configure(prev => ({ ...prev, asyncUtilTimeout: 2000 }))
// Works because prev spreads reactStrictMode — but only if callers remember to spread prev.
// The plain-object form has no such safety net.

configForRTL = {
  ...configForRTL,
  ...(reactStrictMode !== undefined && {reactStrictMode}),
}