fix(router-core): clear leftoverHtml after flushing closing tags to prevent repeated SSR injections

[ali-idrizi]

fixes #5992

Summary by CodeRabbit

• Bug Fixes
  • Fixed a server-side rendering issue where HTML content wasn't properly reset during stream processing, potentially causing rendering problems with routed content.

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[coderabbitai]

Walkthrough

The change modifies the SSR stream transformation logic to reset the leftoverHtml buffer to an empty string after processing chunks containing closing tags, preventing leftoverHtml from accumulating across iterations and causing duplicate query dehydration in streamed responses.

Changes

Cohort / File(s)	Summary
SSR Stream Leftover HTML Buffer Reset packages/router-core/src/ssr/transformStreamWithRouter.ts	Adds logic to reset leftoverHtml to empty string immediately after writing chunks with closing tags (lastIndex > 0), preventing accumulation and duplicate query dehydration across subsequent stream iterations.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

• Single file affected with a targeted variable-reset logic addition
• Clear conditional change addressing a specific accumulation issue
• Low complexity but critical impact on query dehydration during streaming

Possibly related PRs

• fix: cleanup references #5965: Also modifies transformStreamWithRouter.ts and handles cleanup/buffer management in router SSR HTML injection logic
• fix: memory leaks #5896: Modifies stream handling in the same SSR file, restructuring passthrough and injected-HTML listener lifecycle

Suggested labels

package: router-core

Suggested reviewers

• schiller-manuel
• birkskyum

Poem

🐰 A leftover feast no longer grows,
Each closing tag now clearly shows—
Reset the buffer, clear the slate,
Queries dehydrate just once, not eight! ✨
Stream flows smooth through Router's gate.

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: clearing leftoverHtml after flushing closing tags to prevent repeated SSR injections, which directly addresses the issue of queries being dehydrated multiple times.
Linked Issues check	✅ Passed	The code change directly addresses issue #5992 by resetting leftoverHtml after processing closing tags, preventing accumulation that caused repeated query dehydrations during streaming SSR.
Out of Scope Changes check	✅ Passed	All changes are scoped to fixing the leftoverHtml accumulation issue in the SSR streaming logic, with no unrelated modifications present.

✨ Finishing touches

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

---

📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between af02265 and fe2c308.

📒 Files selected for processing (1)

• packages/router-core/src/ssr/transformStreamWithRouter.ts (1 hunks)

🧰 Additional context used

📓 Path-based instructions (1)

**/*.{ts,tsx}

📄 CodeRabbit inference engine (AGENTS.md)

Use TypeScript strict mode with extensive type safety throughout the codebase

Files:

• packages/router-core/src/ssr/transformStreamWithRouter.ts

🧠 Learnings (4)

📚 Learning: 2025-11-25T00:18:21.282Z

Learnt from: CR
Repo: TanStack/router PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-11-25T00:18:21.282Z
Learning: Applies to packages/solid-router/**/*.{ts,tsx} : Solid Router components and primitives should use the tanstack/solid-router package

Applied to files:

• packages/router-core/src/ssr/transformStreamWithRouter.ts

📚 Learning: 2025-10-08T08:11:47.088Z

Learnt from: nlynzaad
Repo: TanStack/router PR: 5402
File: packages/router-generator/tests/generator/no-formatted-route-tree/routeTree.nonnested.snapshot.ts:19-21
Timestamp: 2025-10-08T08:11:47.088Z
Learning: Test snapshot files in the router-generator tests directory (e.g., files matching the pattern `packages/router-generator/tests/generator/**/routeTree*.snapshot.ts` or `routeTree*.snapshot.js`) should not be modified or have issues flagged, as they are fixtures used to verify the generator's output and are intentionally preserved as-is.

Applied to files:

• packages/router-core/src/ssr/transformStreamWithRouter.ts

📚 Learning: 2025-10-01T18:30:26.591Z

Learnt from: schiller-manuel
Repo: TanStack/router PR: 5330
File: packages/router-core/src/router.ts:2231-2245
Timestamp: 2025-10-01T18:30:26.591Z
Learning: In `packages/router-core/src/router.ts`, the `resolveRedirect` method intentionally strips the router's origin from redirect URLs when they match (e.g., `https://foo.com/bar` → `/bar` for same-origin redirects) while preserving the full URL for cross-origin redirects. This logic should not be removed or simplified to use `location.publicHref` directly.

Applied to files:

• packages/router-core/src/ssr/transformStreamWithRouter.ts

📚 Learning: 2025-11-25T00:18:21.282Z

Learnt from: CR
Repo: TanStack/router PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-11-25T00:18:21.282Z
Learning: Applies to packages/react-router/**/*.{ts,tsx} : React Router components and hooks should use the tanstack/react-router package

Applied to files:

• packages/router-core/src/ssr/transformStreamWithRouter.ts

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (2)

• GitHub Check: Preview
• GitHub Check: Test

🔇 Additional comments (3)

packages/router-core/src/ssr/transformStreamWithRouter.ts (3)

241-241: Critical fix for duplicate SSR injections looks correct.

This change directly addresses the bug reported in #5992. Previously, leftoverHtml was written at line 237 but never cleared, causing the same dehydrated query data to be injected multiple times when subsequent chunks contained closing tags. Resetting it here prevents accumulation across iterations and ensures each query is dehydrated exactly once.

---

227-245: Buffer management logic is now consistent across all branches.

The chunk processing correctly handles leftoverHtml in both branches:

• When closing tags are found (lines 233-241): writes accumulated leftoverHtml and clears it
• When no closing tags (lines 242-245): accumulates router stream data into leftoverHtml for the next iteration

This mirrors the pattern used by getBufferedRouterStream() and ensures proper cleanup. The final flush at line 176 correctly includes any remaining leftoverHtml.

---

211-225: Based on my analysis of the review comment provided, I can see that the scratchpad within the comment itself already contains thorough verification of the leftoverHtml handling behavior. The analysis concludes that while the code flow is subtle, it is correct and intentional.

The code properly preserves leftoverHtml through this flow:

1. When body/html end is detected → early exit with partial write (lines 219-221) and leftover = '' (line 223)
2. Later, onEnd is called → final flush (lines 172-178)
3. Final flush writes: leftoverHtml + remaining data + closingTags

This ensures leftoverHtml content appears between the partial content write and the closing tags, which is the correct positioning.

The concern flagged in the review is not an actual bug—it's a subtle but correct implementation pattern. The verification in the existing scratchpad analysis confirms the intended behavior is working as designed.

The leftoverHtml handling is correct and requires no code changes.

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☁️ Nx Cloud last updated this comment at 2025-11-29 12:27:21 UTC

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commit: fe2c308