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Experimental.InlineCss Duplicates The Global Stylesheet Three Times Per Prerendered Page Instead Of The Expected Two

With experimental.inlineCss enabled, the same global CSS text appears three times in prerendered HTML documents: once as a <style> tag and twice in the RSC flight payload. The full flight segment (_full.segment.rsc) re-serializes the hoistable stylesheet instead of reusing the already-emitted row, causing roughly 21% larger prerendered pages and unnecessary ISR write units.

highConfidence 72%Next.js Affected V16.3.1 Affected V16.4.0-Canary.12

Origin Analysis

When assembling the full flight payload (_full.segment.rsc), Next.js fails to deduplicate hoistable stylesheet rows. The leaf segment RSC already contains the global CSS; during full-tree render, the same hoistable style row is emitted again, producing two identical style rows and two identical CSS text rows.

1. Clone the repro repository: https://github.com/TulmansMeubelen/next-inlinecss-flight-duplication-repro 2. Run `npm install` 3. Run `npm run build` (this runs `next build --turbopack`) 4. Run `node count.mjs` to count occurrences of the CSS marker `@layer properties` in the build artifacts. Observe that `nl.html` contains 3 copies, and both `nl.rsc` and `nl.segments/_full.segment.rsc` contain 2 copies.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This hotfix relies on the current structure of RSC flight rows and the exact format of hoistable style elements (`["$","style",...]`). Future changes to Next.js internal row serialization or CSS handling may break the deduplication logic, potentially leaving orphaned rows or removing non-CSS data. The implementation processes the entire payload as a string, which is acceptable for static generation but may increase memory consumption for very large applications. Test thoroughly with Tailwind and other CSS-in-JS setups before deploying. Rollback: remove the dedupe call and revert to the original payload generation; no changes are made to SSR `<style>` generation, so HTML output remains unaffected.