fix(react-start-client): preserve idle hydration deadlines

[LadyBluenotes]

🎯 Changes

Inline idle() calls create new strategy objects on each render, which restarted pending hydration. Keep the existing schedule when its timeout is unchanged, while preserving timeout and strategy changes, custom setup overrides, and unmount cleanup.

✅ 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).

Summary by CodeRabbit

• Bug Fixes

  • Improved idle hydration reliability during parent component re-renders.
  • Pending hydration now remains scheduled when the idle timeout is unchanged.
  • Hydration is correctly rescheduled when the timeout or strategy changes.
  • Pending idle hydration is cancelled when a component is unmounted.

• Documentation

  • Clarified how recreating idle hydration settings affects scheduling and replacement behavior.

[nx-cloud]

View your CI Pipeline Execution ↗ for commit 81fa70d

Command	Status	Duration	Result
nx affected --targets=test:eslint,test:unit,tes...	✅ Succeeded	19m 45s	View ↗
nx run-many --target=build --exclude=examples/*...	✅ Succeeded	1m 43s	View ↗

---

☁️ Nx Cloud last updated this comment at 2026-09-06 18:53:50 UTC

[github-actions]

🚀 Changeset Version Preview

9 package(s) bumped directly, 16 bumped as dependents.

🟩 Patch bumps

Package	Version	Reason
@tanstack/history	1.162.1 → 1.162.2	Changeset
@tanstack/nitro-v2-vite-plugin	1.155.0 → 1.155.1	Changeset
@tanstack/react-router	1.170.32 → 1.170.33	Changeset
@tanstack/react-start-client	1.168.30 → 1.168.31	Changeset
@tanstack/router-core	1.171.27 → 1.171.28	Changeset
@tanstack/solid-router	1.170.30 → 1.170.31	Changeset
@tanstack/start-client-core	1.170.27 → 1.170.28	Changeset
@tanstack/start-plugin-core	1.171.39 → 1.171.40	Changeset
@tanstack/vue-router	1.170.29 → 1.170.30	Changeset
@tanstack/react-start	1.168.49 → 1.168.50	Dependent
@tanstack/react-start-rsc	0.1.48 → 0.1.49	Dependent
@tanstack/react-start-server	1.167.37 → 1.167.38	Dependent
@tanstack/router-cli	1.167.33 → 1.167.34	Dependent
@tanstack/router-generator	1.167.33 → 1.167.34	Dependent
@tanstack/router-plugin	1.168.35 → 1.168.36	Dependent
@tanstack/router-vite-plugin	1.167.35 → 1.167.36	Dependent
@tanstack/solid-start	1.168.47 → 1.168.48	Dependent
@tanstack/solid-start-client	1.168.29 → 1.168.30	Dependent
@tanstack/solid-start-server	1.167.36 → 1.167.37	Dependent
@tanstack/start-server-core	1.169.31 → 1.169.32	Dependent
@tanstack/start-static-server-functions	1.167.32 → 1.167.33	Dependent
@tanstack/start-storage-context	1.167.29 → 1.167.30	Dependent
@tanstack/vue-start	1.168.46 → 1.168.47	Dependent
@tanstack/vue-start-client	1.167.32 → 1.167.33	Dependent
@tanstack/vue-start-server	1.167.36 → 1.167.37	Dependent

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📥 Commits

Reviewing files that changed from the base of the PR and between 216c0c4 and 81fa70d.

📒 Files selected for processing (6)

• .changeset/idle-hydration-deadline.md
• docs/start/framework/react/guide/deferred-hydration.md
• packages/react-start-client/src/GenericHydrate.tsx
• packages/react-start-client/src/tests/Hydrate.test.tsx
• packages/start-client-core/src/hydration/idle.ts
• packages/start-client-core/src/hydration/types.ts

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---

📝 Walkthrough

Walkthrough

Idle hydration now preserves pending work when its timeout remains unchanged. It reschedules when the timeout or strategy changes, cancels work on unmount, and documents and tests these behaviors.

Changes

Idle hydration scheduling

Layer / File(s)	Summary
Idle strategy identity contract packages/start-client-core/src/hydration/types.ts, packages/start-client-core/src/hydration/idle.ts	The idle setup function now exposes its normalized timeout through _i. The hydration strategy type supports this optional property.
React hydration scheduling integration packages/react-start-client/src/GenericHydrate.tsx	React hydration tracks the latest strategy and uses a stable strategy key and strategy type to control effect execution.
Scheduling behavior validation and documentation packages/react-start-client/src/tests/Hydrate.test.tsx, docs/start/framework/react/guide/deferred-hydration.md, .changeset/idle-hydration-deadline.md	Tests cover deadline preservation, timeout changes, custom setup replacement, and unmount cancellation. Documentation and release metadata describe the behavior.

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

Merge Risk: ⚪ Minimal · up to 81fa7

React idle hydration now retains pending work across rerenders when the timeout is unchanged while preserving rescheduling and cleanup behavior. The covered behavior is ready to merge.

Sequence Diagram(s)

sequenceDiagram
  participant GenericHydrate
  participant HydrationStrategy
  participant idleSetup
  participant BrowserScheduler
  GenericHydrate->>HydrationStrategy: Read current strategy and stable key
  GenericHydrate->>idleSetup: Invoke _s setup
  idleSetup->>BrowserScheduler: Schedule idle hydration
  BrowserScheduler-->>GenericHydrate: Run hydration callback
  GenericHydrate->>GenericHydrate: Cancel setup on replacement or unmount

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ 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%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 4 files. (2 skipped: 2 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description follows the repository template. It explains the change, completes all checklist items, and documents the release impact and changeset.
Title check	✅ Passed	The title clearly and concisely describes the primary change: preserving idle hydration deadlines in react-start-client.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 4 files. (2 skipped: 2 unsupported.)

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/react-idle-hydration-deadline

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[github-actions]

Bundle Size Benchmarks

This pull request does not affect bundle size in any measured scenario.

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commit: 81fa70d

[codspeed-hq]

Merging this PR will not alter performance

✅ 180 untouched benchmarks

---

_{Comparing fix/react-idle-hydration-deadline (81fa70d) with main (216c0c4)}