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

Bug

Calling configure() with any config object that omits reactStrictMode silently resets the previously-configured strict mode flag to undefined.

Root cause

In src/config.js:

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

  const {reactStrictMode, ...configForDTL} = newConfig

  configureDTL(configForDTL)

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

When newConfig does not include a reactStrictMode key, destructuring yields reactStrictMode = undefined. The spread ...configForRTL correctly carries the previously saved value, but the explicit reactStrictMode property after it overwrites it with undefined.

Reproduction

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)

The second configure() call erases the strict-mode setting. The function-form callback is affected identically:

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.

Impact

Tests that rely on global strict-mode configuration (e.g. set in a setupFilesAfterFramework) silently lose that configuration if any later configure() call omits reactStrictMode. This is particularly easy to hit when test utilities or libraries call configure() with their own options.

Fix

Only write reactStrictMode into configForRTL when it is explicitly present in newConfig:

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

PR #1461 implements exactly this fix.

Start in src/config.js at configure(), then review how getConfig() exposes the saved reactStrictMode value. The fix is done when a later configure() call that omits reactStrictMode preserves the prior setting, including the plain-object case described in the reproduction.