fix(svelte-query): shrink useMutationState results

[alex-js-ltd]

Changes

Fixes#11152.

useMutationState() in @tanstack/svelte-query only ever grew its returned
array. The cache-subscription callback updated the underlying $state array
with Object.assign(result, nextResult), but Array.prototype.length is
non-enumerable, so Object.assign never truncates result when the new
result set is shorter. Once a mutation stopped matching filters (settled,
was removed/GC'd, or a narrower filter was applied), its stale entry stayed
in the array forever.

Fix: replace the Object.assign with result.splice(0, result.length, ...nextResult),
which replaces the array's contents in place — including truncating it —
while preserving the $state array's reference identity (required by the rune).

Includes a regression test that starts two mutations matching
filters: { status: 'pending' }, lets both settle, and asserts the returned
array goes back to [] instead of keeping the two stale "pending" entries.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 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

  • Fixed useMutationState so mutations are removed from filtered results when they no longer match the selected criteria.
  • Improved reactive updates to ensure mutation state results stay accurate as statuses change.

• Tests

  • Added coverage for removing mutations that no longer match a pending-status filter.

[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: eef56c9e-f49e-4db7-82ff-5d61602392ad

📥 Commits

Reviewing files that changed from the base of the PR and between 46d7f02 and 9f71f4f.

📒 Files selected for processing (3)

• .changeset/svelte-query-usemutationstate-filter-shrink.md
• packages/svelte-query/src/useMutationState.svelte.ts
• packages/svelte-query/tests/useMutationState/useMutationState.svelte.test.ts

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📝 Walkthrough

Walkthrough

useMutationState now removes mutations that no longer match its filters. A regression test covers pending mutations after completion, and a patch changeset documents the fix.

Changes

Mutation filter result updates

Layer / File(s)	Summary
Replace stale filtered results packages/svelte-query/src/useMutationState.svelte.ts, packages/svelte-query/tests/useMutationState/useMutationState.svelte.test.ts, .changeset/svelte-query-usemutationstate-filter-shrink.md	The reactive result array now uses splice to match the current filtered results. The test verifies that completed pending mutations produce an empty result. A patch changeset records the fix.

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

Possibly related PRs

• TanStack/query#11123: Both PRs modify useMutationState tests, but this PR fixes stale filtered results.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the primary fix: shrinking filtered useMutationState results.
Description check	✅ Passed	The description explains the bug, fix, regression test, checklist completion, and release impact with a changeset.
Linked Issues check	✅ Passed	The implementation, regression test, and changeset satisfy the requirements in issue #11152.
Out of Scope Changes check	✅ Passed	All changes directly support fixing filtered useMutationState results and adding regression coverage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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