[Turbopack] Excessive Fast Refresh rebuilds when dev server runs on Bun runtime

Link to the code that reproduces this issue

https://github.com/MrScotch679/NextBunFastRefresh

To Reproduce

Environment

• Next: 16.1.1
• Bun: 1.3.8
• OS: Windows
• Dev command: bun --bun next dev --turbopack ...
• Bundler: Turbopack
• With Node runtime (node ./node_modules/next/dist/bin/next dev --turbopack): no issue
• With Webpack (bun --bun next dev --webpack): no issue, but slower

Symptom

• Single file save can trigger many [Fast Refresh] rebuilding / [Fast Refresh] done in … pairs in the browser console.
• The number of cycles tends to grow over time and sometimes increases after page reload.

Expected: one BUILDING/BUILT pair per save (or at most a couple).

Actual: multiple pairs per save, with the count increasing on larger apps and over time.

Reproduction

Small app (minimal):

1. Create a minimal Next app.
2. In package.json set the dev script to bun --bun next dev --turbopack.
3. Run bun run dev, open / in the browser.
4. Edit src/app/page.tsx (e.g. add a space) and save.
5. You may see more than one pair of [Fast Refresh] rebuilding / [Fast Refresh] done for that single save; on very small apps the behavior is not always stable.

Large app (stable repro):

1. Use an App Router app (many modules/routes, global SCSS entry, next-intl with dynamic message imports, large barrel, layouts importing env and shared exports).
2. Run bun --bun next dev --turbopack, open any page.
3. Edit a single file and save.
4. You should see multiple [Fast Refresh] rebuilding / done pairs (often 5 or more); after several such edits, the number of pairs per save tends to increase further.

Project characteristics

• Even on small apps with Bun + Turbopack, a single save can produce multiple [Fast Refresh] rebuilding/done pairs.
• On larger apps, the number of pairs per save increases markedly and can grow over the dev session.

What works / doesn’t

• Node + Turbopack → OK
• Bun + Webpack → OK (slower)
• Bun + Turbopack → multiple BUILDING/BUILT (Fast Refresh) per change; effect worsens with project size.

Current vs. Expected behavior

Expected: one BUILDING/BUILT pair per save (or at most a couple).

Actual: multiple pairs per save, with the count increasing on larger apps and over time.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Pro
  Available memory (MB): 65378
  Available CPU cores: 28
Binaries:
  Node: 22.14.0
  npm: 11.5.2
  Yarn: N/A
  pnpm: 10.11.0
Relevant Packages:
  next: 16.1.4 // There is a newer version (16.1.6) available, upgrade recommended!
  eslint-config-next: 15.5.4
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A
⚠ There is a newer version (16.1.6) available, upgrade recommended!
   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)

Not sure

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

next dev (local)

Additional context

Related

Bun: https://github.com/oven-sh/bun/issues/18113

Start with the linked NextBunFastRefresh reproduction, package.json, and the src/app/page.tsx edit described in the report. Run bun --bun next dev --turbopack and compare one save with Node + Turbopack and Bun + Webpack; done means a save produces one Fast Refresh rebuilding/done pair rather than repeated cycles.