fix(svelte-query): useMutationState array never shrank when mutations stopped matching filter

[okxint]

Summary

useMutationState() in @tanstack/svelte-query never removed entries from its returned array. Once a mutation stopped matching filters (settled, was garbage-collected, or mutationCache.clear() was called), the stale entry stayed in the array forever.

Root cause

The cache-subscription callback updates the reactive $state array with:

Object.assign(result, nextResult)

Object.assign copies enumerable own properties — the numeric indices of nextResult — but never adjusts Array.length. When nextResult is shorter than result, the trailing stale entries are never touched and result.length is never updated.

This bug was introduced in the Svelte 5 runes rewrite (#9694) and has been present since.

Fix

- Object.assign(result, nextResult)
+ result.splice(0, result.length, ...nextResult)

splice keeps the $state array reference intact (required by Svelte 5 runes — reassigning result = nextResult would lose reactivity) while correctly adjusting length, replacing all elements, and removing stale trailing entries.

Test

Added regression test: two mutations started concurrently both match filters: { status: 'pending' }. After both settle, the result array must shrink to []. Previously this assertion failed — the array stayed ["pending", "pending"] indefinitely.

All 7 existing tests continue to pass.

Fixes #11152

Summary by CodeRabbit

• Bug Fixes

  • Fixed mutation state tracking so results are removed when mutations no longer match the selected filter.
  • Ensured filtered mutation lists accurately shrink after mutations finish.

• Tests

  • Added coverage verifying pending mutation entries are cleared once mutations settle.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: b76c5445-8b9c-4670-99a4-2953482de0fd

📥 Commits

Reviewing files that changed from the base of the PR and between 2215bb0 and 9002ee6.

📒 Files selected for processing (3)

• .changeset/fix-svelte-use-mutation-state-shrink.md
• packages/svelte-query/src/useMutationState.svelte.ts
• packages/svelte-query/tests/useMutationState/useMutationState.svelte.test.ts

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

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📝 Walkthrough

Walkthrough

useMutationState now updates its reactive array with splice, which removes mutations that no longer match the filter. A regression test verifies the array changes from two pending mutations to an empty array after settlement. A Changesets file declares a patch release.

Changes

Mutation state shrink fix

Layer / File(s)	Summary
Synchronize filtered mutation results packages/svelte-query/src/useMutationState.svelte.ts, packages/svelte-query/tests/useMutationState/useMutationState.svelte.test.ts	The result array now uses splice to replace its contents. The test verifies that settled mutations are removed from a pending-only result.
Record the patch release .changeset/fix-svelte-use-mutation-state-shrink.md	Adds patch release metadata for @tanstack/svelte-query.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 9002e

This localized change corrects stale mutation-state entries and adds regression coverage; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 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 1 functions across 2 files. (1 skipped: 1 unsupported.)	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the Svelte Query bug and the array-shrinking fix.
Description check	✅ Passed	The description clearly explains the bug, root cause, fix, regression test, and linked issue, with only template headings and checklist items omitted.
Linked Issues check	✅ Passed	The source fix and regression test directly satisfy issue #11152 by removing stale filtered mutations while preserving reactive array identity.
Out of Scope Changes check	✅ Passed	The changeset, source fix, and regression test all directly support issue #11152 and contain no unrelated changes.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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