Link to the code that reproduces this issue
https://github.com/TulmansMeubelen/next-inlinecss-flight-duplication-repro
To Reproduce
npm installnpm run build (next build --turbopack)node count.mjs — counts occurrences of the CSS marker (@layer properties) in the build artifacts
The repro is intentionally tiny: a root layout importing one global stylesheet, one app/[locale]/page.tsx with generateStaticParams, and experimental: { inlineCss: true }.
Current vs. Expected behavior
Current (both 16.3.1 and 16.4.0-canary.12):
| artifact |
copies of the CSS text |
.next/server/app/nl.html |
3 (1 <style> + 2 flight rows) |
.next/server/app/nl.rsc |
2 |
.next/server/app/nl.segments/_full.segment.rsc |
2 |
.next/server/app/nl.segments/_index.segment.rsc |
1 |
| every other segment file |
0 |
The two flight copies are byte-identical T-rows, each referenced by its own hoistable style element row (["$","style","1",{"precedence":"next","href":"<chunk>.css","children":"$<id>"}]). Both land in the first flushes of the stream. Since the individual segments together only carry one copy, the second copy appears when the full flight (_full.segment.rsc) is assembled — the assembly re-serializes the hoistable stylesheet instead of reusing the already-emitted row.
Expected: at most one copy of the CSS text in the flight payload — the inlineCss docs describe exactly one RSC-payload copy as the known trade-off ("Styles are duplicated during initial page load - once within <style> tags for SSR and once in the RSC payload"), i.e. 2 copies per document, not 3.
Provide environment information
Operating System:
Platform: win32
Arch: x64
Version: Windows 11 Pro
Available memory (MB): 32679
Available CPU cores: 16
Binaries:
Node: 24.11.1
npm: 11.6.2
Relevant Packages:
next: 16.4.0-canary.12 (also reproduced on 16.3.1)
react: 19.2.8
react-dom: 19.2.8
typescript: 7.0.2
Which area(s) are affected? (Select all that apply)
CSS, Turbopack, Runtime
Additional context
On a real Tailwind application the inlined stylesheet is large (ours is 249 KB), so the extra flight copy is expensive: every prerendered/ISR-written HTML document carries ~500 KB of flight CSS on top of the 255 KB <style> block. On Vercel this roughly triples ISR write units and fast origin transfer for HTML documents (we measured a content page at 1,174 KB with the flag vs 415 KB without; the third copy alone is ~250 KB per page). Deduplicating the flight copy would remove ~21% of every prerendered page at no functional cost.
Link to the code that reproduces this issue
https://github.com/TulmansMeubelen/next-inlinecss-flight-duplication-repro
To Reproduce
npm installnpm run build(next build --turbopack)node count.mjs— counts occurrences of the CSS marker (@layer properties) in the build artifactsThe repro is intentionally tiny: a root layout importing one global stylesheet, one
app/[locale]/page.tsxwithgenerateStaticParams, andexperimental: { inlineCss: true }.Current vs. Expected behavior
Current (both
16.3.1and16.4.0-canary.12):.next/server/app/nl.html<style>+ 2 flight rows).next/server/app/nl.rsc.next/server/app/nl.segments/_full.segment.rsc.next/server/app/nl.segments/_index.segment.rscThe two flight copies are byte-identical
T-rows, each referenced by its own hoistable style element row (["$","style","1",{"precedence":"next","href":"<chunk>.css","children":"$<id>"}]). Both land in the first flushes of the stream. Since the individual segments together only carry one copy, the second copy appears when the full flight (_full.segment.rsc) is assembled — the assembly re-serializes the hoistable stylesheet instead of reusing the already-emitted row.Expected: at most one copy of the CSS text in the flight payload — the
inlineCssdocs describe exactly one RSC-payload copy as the known trade-off ("Styles are duplicated during initial page load - once within<style>tags for SSR and once in the RSC payload"), i.e. 2 copies per document, not 3.Provide environment information
Which area(s) are affected? (Select all that apply)
CSS, Turbopack, Runtime
Additional context
On a real Tailwind application the inlined stylesheet is large (ours is 249 KB), so the extra flight copy is expensive: every prerendered/ISR-written HTML document carries ~500 KB of flight CSS on top of the 255 KB
<style>block. On Vercel this roughly triples ISR write units and fast origin transfer for HTML documents (we measured a content page at 1,174 KB with the flag vs 415 KB without; the third copy alone is ~250 KB per page). Deduplicating the flight copy would remove ~21% of every prerendered page at no functional cost.