reactStrictMode is not effect in nextjs v15.x, but it's work in nextjs v14

[takeseem]

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Node.js v23.6.0

Operating System:
  Platform: linux
  Arch: x64
  Version: #52-Ubuntu SMP PREEMPT_DYNAMIC Thu Dec  5 13:09:44 UTC 2024
  Available memory (MB): 92316
  Available CPU cores: 16
Binaries:
  Node: 23.6.0
  npm: 10.9.2
  Yarn: N/A
  pnpm: 9.15.4
Relevant Packages:
  next: 15.2.0-canary.12 // Latest available version is detected (15.2.0-canary.12).
  eslint-config-next: 14.2.23
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.7.3
Next.js Config:
  output: N/A

Which example does this report relate to?

/~https://github.com/takeseem/demo-nextjs-v14/commits/main/

What browser are you using? (if relevant)

chrome 131.0

How are you deploying your application? (if relevant)

pnpm dev

Describe the Bug

• pnpx create-next-app@14.2.23
• add react StictMode test code in the Home page src/app/page.tsx

  useEffect(() => {
    console.log(new Date().toLocaleString(), "App mounted");
    return () => {
      console.log(new Date().toLocaleString(), "App umounted");
    }
  }, []);

• it's work

---

• test in nextjs v15.x and v15. canary is fail
• test code

Expected Behavior

• reactStrictMode: true work in v15

To Reproduce

• git clone git@github.com:takeseem/demo-nextjs-v14.git
• cd demo-nextjs-v14 && pnpm i && pnpm dev
• http://localhost:3000
• check dev tool console out like this