Next / React 19.2 suspense boundary outlining bug

Why this is a good first issue

This issue involves React 19.2 suspense boundary SSR behavior in Next.js, which may require deep React internals knowledge.

AI Summary

The issue reproduces a React 19.2 suspense boundary SSR bug in Next.js, where fallback content is incorrectly rendered in server HTML. The fix likely requires understanding React's SSR behavior and suspense boundaries, potentially involving upstream React changes.

Issue Description

Link to the code that reproduces this issue

https://github.com/jaslong/next-react-19.2-suspense-boundary-ssr

To Reproduce

Run the dev server and open http://localhost:3000 (app router) or https://localhost:3000/pages (pages router) with devtools open. You can see the server HTML in the network tab of the GET request or with JavaScript disabled.

Current vs. Expected behavior

This repo reproduces a React 19.2 suspense boundary SSR issue in Next.js facebook/react issue #35460. You'll see suspense boundaries containing large HTML content that can't possibly throw/suspend, yet the fallback is rendered in the server HTML.

Provide environment information

npx next info                                                                                                                                                                                                                                                                   main 640174a

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.3.0: Wed Jan 28 20:54:55 PST 2026; root:xnu-12377.91.3~2/RELEASE_ARM64_T6031
  Available memory (MB): 36864
  Available CPU cores: 14
Binaries:
  Node: 24.4.0
  npm: 11.4.2
  Yarn: 1.22.22
  pnpm: 10.8.0
Relevant Packages:
  next: 16.2.1-canary.4 // There is a newer canary version (16.2.1-canary.5) available, please upgrade!
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A
⚠ There is a newer canary version (16.2.1-canary.5) available, please upgrade!
   Please try the latest canary version (`npm install next@canary`) to confirm the issue still exists before creating a new issue.
   Read more - https://nextjs.org/docs/messages/opening-an-issue

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

React

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

next dev (local), next build (local), next

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