fix(start): keep dev SSR styles until HMR makes them stale by ViliamKopecky · Pull Request #8767 · TanStack/router

ViliamKopecky · 2026-10-19T11:23:41Z

🎯 Changes

Written with Claude Code (AI); reviewed and submitted by @ViliamKopecky.

Dev only: a React.lazy component rendered during SSR flashes unstyled after hydration. /@tanstack-start/styles.css contains its CSS, but HeadContent.dev drops the link on hydration while the lazy module is still loading.

 HeadContent.dev (react / solid / vue)
-  remove dev styles link when hydrated
+  remove it via onDevStylesStale(...)        (runs on mount if already stale)

 hydrateStartWithHmr (start-client-core)
+  hot.on?.('vite:afterUpdate', markDevStylesStale)

 router-core/manifest.ts
+  markDevStylesStale / onDevStylesStale      (module state)

The link only goes stale after an edit, so HMR is the removal point. The listener is in start-client-core because react-router also builds CJS (import.meta not allowed). Rsbuild dev doesn't emit the dev styles link, so only Vite is wired.

New e2e/react-start/dev-ssr-styles/tests/lazy-ssr.spec.ts:

block LazySsrPanel.tsx → hydrate → panel stays styled       (main: "rgba(0, 0, 0, 0)")
HMR-delete a declaration → gone, same document (no reload)
HMR edit while hydration is held back → link dropped once HeadContent mounts

Chunk-blocking tests are skipped in bundled-dev (the chunk can't be held back). benchmark:bundle-size:run vs main: +0 bytes in all 18 scenarios.

Not covered: a Solid/Vue lazy-SSR repro (their existing suites pass). DevTools shows SSR rules twice until the first edit.

✅ Checklist

I have followed the steps in the Contributing guide.
I have tested code changes locally with the relevant test commands, or tests do not apply to this pull request.
I fully understand the code in this pull request, including any code generated with AI assistance.

🚀 Release Impact

This change affects published code, and I have generated a changeset.
This change is docs/CI/dev-only (no release).

🤖 Generated with Claude Code

Summary by CodeRabbit

Bug Fixes
- Development SSR styles now remain applied after hydration while lazy content is loading, and are removed when an HMR update makes them stale.
Tests
- Added coverage for style retention during lazy loading and for style updates after HMR without a page reload.

HeadContent removed the /@tanstack-start/styles.css link as soon as the app hydrated. Server-rendered content inside a Suspense boundary whose module was still loading (e.g. React.lazy) lost its styles until the bundler injected them, causing a flash of unstyled content in dev. Keep the link until start-client-core reports an applied HMR update (the only point where the snapshot can be stale) via a window event. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

…ev repro - React/Solid/Vue register the stale listener on mount and drop the link in the handler, without module-level state. - Skip the lazy-module blocking test in bundled dev, where the lazy chunk is part of the client bundle and cannot be held back. - Mark DEV_STYLES_STALE_EVENT @internal; type Vite's `on` payload. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

changeset-bot · 2026-10-19T11:23:48Z

🦋 Changeset detected

Latest commit: fa11dfc

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 23 packages

Name	Type
@tanstack/router-core	Patch
@tanstack/react-router	Patch
@tanstack/solid-router	Patch
@tanstack/vue-router	Patch
@tanstack/start-client-core	Patch
@tanstack/react-start-client	Patch
@tanstack/react-start-rsc	Patch
@tanstack/react-start-server	Patch
@tanstack/router-generator	Patch
@tanstack/router-plugin	Patch
@tanstack/solid-start-client	Patch
@tanstack/solid-start-server	Patch
@tanstack/start-plugin-core	Patch
@tanstack/start-server-core	Patch
@tanstack/start-storage-context	Patch
@tanstack/vue-start-client	Patch
@tanstack/vue-start-server	Patch
@tanstack/react-start	Patch
@tanstack/solid-start	Patch
@tanstack/vue-start	Patch
@tanstack/start-static-server-functions	Patch
@tanstack/router-cli	Patch
@tanstack/router-vite-plugin	Patch

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coderabbitai · 2026-10-19T11:29:40Z

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packages/router-core/src/manifest.ts
packages/start-client-core/src/client/hydrateStart.ts

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📝 Walkthrough

Router head components retain development SSR styles after hydration. Vite HMR updates mark those styles stale, after which the components remove matching stylesheet links. A React Start example and browser tests cover styles during lazy loading and after HMR updates.

Changes: Development SSR style lifecycle

Layer / File(s)	Summary
Stale-style notification and HMR integration `packages/router-core/src/manifest.ts`, `packages/router-core/src/index.ts`, `packages/start-client-core/src/client/hydrateStart.ts`	Router core tracks development-style staleness and exports notification APIs. Client hydration marks styles stale after Vite's `vite:afterUpdate` event.
Head component handling `packages/react-router/src/HeadContent.dev.tsx`, `packages/solid-router/src/HeadContent.dev.tsx`, `packages/vue-router/src/HeadContent.dev.tsx`	The head components retain development styles after hydration until notified of staleness. They then remove matching stylesheet links and filter development styles from rendered tags.
Lazy SSR example and validation `e2e/react-start/dev-ssr-styles/src/routes/lazy-ssr.tsx`, `e2e/react-start/dev-ssr-styles/src/components/LazySsrPanel.tsx`, `e2e/react-start/dev-ssr-styles/src/styles/*`, `e2e/react-start/dev-ssr-styles/src/routeTree.gen.ts`, `e2e/react-start/dev-ssr-styles/tests/lazy-ssr.spec.ts`, `.changeset/calm-lazy-styles.md`	The example adds a lazy-loaded panel and route. Browser tests check styling during lazy loading and CSS updates after HMR. The changeset declares patch releases for five packages.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: schiller-manuel

Merge Risk: ⚪ Minimal · up to fa11d

The supported development setup shares one router-core instance between HMR and head consumers, so the reported split-module stale-style issue is not present there. No actionable merge blocker remains.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 10 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: retaining development SSR styles until an HMR update makes them stale.
Description check		The description includes all required sections, explains the motivation and implementation, documents testing and limitations, discloses AI authorship, and confirms the changeset. One checklist item a…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
Review comments at @packages/start-client-core/src/client/hydrateStart.ts:
- Around line 67-92: Keep the initial dev-styles stale state false for hydration
consistency, and add a persistent marker that notifyDevStylesStale sets when
dispatching the event. After each framework registers its stale-event listener,
check the marker and invoke that framework’s existing stale handler if set, so
notifications received during pending hydration are applied after mount.

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📒 Files selected for processing: 12

.changeset/calm-lazy-styles.md
e2e/react-start/dev-ssr-styles/src/components/LazySsrPanel.tsx
e2e/react-start/dev-ssr-styles/src/routeTree.gen.ts
e2e/react-start/dev-ssr-styles/src/routes/lazy-ssr.tsx
e2e/react-start/dev-ssr-styles/src/styles/lazy-ssr-panel.module.css
e2e/react-start/dev-ssr-styles/tests/lazy-ssr.spec.ts
packages/react-router/src/HeadContent.dev.tsx
packages/router-core/src/index.ts
packages/router-core/src/manifest.ts
packages/solid-router/src/HeadContent.dev.tsx
packages/start-client-core/src/client/hydrateStart.ts
packages/vue-router/src/HeadContent.dev.tsx

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An HMR update applied while the app is still initializing (e.g. awaiting getRouter) fired before HeadContent mounted, leaving the stale SSR snapshot in place. router-core now keeps the stale flag and exposes internal markDevStylesStale/onDevStylesStale helpers used by Start and the React/Solid/Vue HeadContent. The HMR e2e tests also assert the page was not reloaded. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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Actionable comments posted: 1

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
Review comments at @packages/router-core/src/manifest.ts:
- Line 12: Replace the module-local stale state used by `markDevStylesStale` and
`onDevStylesStale` with a flag stored on `window`, so duplicated module copies
share it. Have `onDevStylesStale` check the shared flag before registering its
event listener, preserving immediate callbacks for late subscribers.

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📒 Files selected for processing: 9

e2e/react-start/dev-ssr-styles/src/routes/lazy-ssr.tsx
e2e/react-start/dev-ssr-styles/src/styles/app.css
e2e/react-start/dev-ssr-styles/tests/lazy-ssr.spec.ts
packages/react-router/src/HeadContent.dev.tsx
packages/router-core/src/index.ts
packages/router-core/src/manifest.ts
packages/solid-router/src/HeadContent.dev.tsx
packages/start-client-core/src/client/hydrateStart.ts
packages/vue-router/src/HeadContent.dev.tsx

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- Keep listeners in router-core module state instead of a window event; the flag and the listeners now live in the same scope. - Drop @internal tags: stripInternal removed the declarations from manifest.d.ts while index.d.ts still re-exported them. - Remove the webpack/rspack status handler: Rsbuild dev never emits the dev styles link. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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fix(start): keep dev SSR styles until HMR makes them stale - #8767

fix(start): keep dev SSR styles until HMR makes them stale#8767
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ViliamKopecky:fix/dev-styles-keep-until-hmr

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