AI & Agent Dev Bug Sandbox Back to Radar

Experimental.InlineCss Duplicates Global Stylesheet 3X Due To Empty InjectedCSS Set In GetGlobalErrorStyles

With experimental.inlineCss enabled, the full global stylesheet is embedded three times in every prerendered page: once in the SSR <style> tag, once in the RSC payload segment seed data (documented limitation), and a third time under the GlobalError entry because getGlobalErrorStyles passes a fresh empty Set as injectedCSS to createComponentStylesAndScripts, causing re-inlining of all root layout styles. This significantly inflates HTML size (e.g., ~1.3MB extra per page for a 435KB Tailwind sheet), increasing parse/memory cost and ISR write units.

highConfidence 85%Next.js Affected V16.2.10 Affected V16.3.3 Affected V16.4.0-Canary.12

Origin Analysis

In packages/next/src/server/app-render/app-render.tsx, getGlobalErrorStyles invokes createComponentStylesAndScripts with injectedCSS: new Set(), ignoring the Set of CSS already inlined during the root layout render. This bypasses deduplication and causes every global stylesheet to be injected again under the GlobalError flight payload, even when no custom global-error.tsx exists.

1. Clone https://github.com/winkelmeyer/next-inlinecss-global-error-duplication 2. Run npm install 3. Run npx next build 4. Run grep -c -- '--repro-marker-inlinecss-dup' .next/server/app/index.html Observe the marker appears 3 times, indicating three full copies of the global stylesheet.

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

The fix assumes rootLayoutInjectedCSS is the exact Set used for root layout in this request context and is not cleared or replaced before getGlobalErrorStyles runs. If the Set is shared across concurrent requests or mutated asynchronously during streaming, it could lead to race conditions or missing styles. Ensure the Set is per-request. The change only eliminates the third copy; the documented second copy in the RSC payload remains. Rollback by reverting to new Set() if unexpected missing CSS occurs in global-error boundaries.