nextjs - 💡(How to fix) Fix router.push inside useActionState action body breaks Back button when proxy.ts emits Set-Cookie (16.2.x regression) [1 participants]

Link to the code that reproduces this issue

https://github.com/jncampbell/nextjs-repro-back-button-16.2

To Reproduce

1. git clone https://github.com/jncampbell/nextjs-repro-back-button-16.2
2. cd nextjs-repro-back-button-16.2
3. npm install
4. npm run build && npm start
5. Open http://localhost:3000/account in a fresh browser tab (so the only entry in browser history is the new-tab page).
6. Click Submit. URL changes to /dob.
7. Click the browser Back button.

Current vs. Expected behavior

Expected: Back returns to /account.

Actual: Back lands on the new-tab page (the entry before /account). The /account history entry has been clobbered — replaceState was used instead of pushState when navigating to /dob.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.3.0: Wed Jan 28 20:53:05 PST 2026; root:xnu-12377.81.4~5/RELEASE_ARM64_T6020
  Available memory (MB): 32768
  Available CPU cores: 12
Binaries:
  Node: 22.0.0
  npm: 10.5.1
  Yarn: 4.9.1
  pnpm: N/A
Relevant Packages:
  next: 16.2.3
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Linking and Navigating, Server Actions, Middleware

Which stage(s) are affected? (Select all that apply)

Other (Deployed), next build (local), next start (local)

Additional context

This is the same class of regression as #90513, partially fixed by #90533. The fix in #90533 does not cover the case where proxy.ts emits a Set-Cookie header and router.push is called from inside a useActionState action body — both of which are common patterns in production apps (any auth proxy, any cookie-based session, any "submit form then navigate" flow).

The minimal failing form looks like:

const [, formAction] = useActionState(async (prev, fd) => {
    const result = await someServerAction(prev, fd);   // await suspends the transition
    if (result?.redirectPath) router.push(result.redirectPath);  // runs while suspended
    return result;
 }, null);

return <form action={formAction}>…</form>;

Tested against canary branch and can still reproduce