perf(useSlots): optimize slot matching with short-circuit and reduced allocations

[hectahertz]

Closes #

Optimizes the useSlots hook used by 8+ components (ActionList.Item, ActionList, FormControl, TreeView, NavList, PageLayout, Dialog, CheckboxOrRadioGroup). The hook iterates children on every render to extract slot components.

Changes:

• Production short-circuit: once all slots are filled, remaining children go directly to rest with zero type checking (single integer comparison). Duplicate detection only runs in __DEV__ and is stripped in production builds.
• for loop instead of findIndex: eliminates closure allocation per child per render
• for loop init instead of mapValues+Object.keys().reduce(): replaces a higher-order reduce with a simple loop to initialize slot keys

Metrics

Microbenchmark (Chrome, 100K iterations, JIT-warmed):

Operation	Old	New	Speedup
Slot init (mapValues+reduce vs for loop)	0.05 us	0.04 us	20%
Child matching (findIndex+closure vs for loop)	0.07 us	0.05 us	31%
Full useSlots call	0.21 us	0.14 us	✨ 33% ✨

Stress test (ActionList, 100 items x 100 re-renders):

Metric	main	Optimized	Change
DevTools trace duration (100 re-renders)	19,014 ms	17,891 ms	-6%
Story-reported median	7.23 ms	6.70 ms	-7%
Story-reported average	5.42 ms	4.37 ms	-19%

Real-world impact is larger than this test shows: 8+ components call useSlots, production builds strip the __DEV__ duplicate-detection loop.

Changelog

New

N/A

Changed

• useSlots: optimized child iteration with production short-circuit and reduced allocations

Removed

N/A

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

1. All 13 original useSlots unit tests pass unmodified (behavior-preserving)
2. 4 new tests added covering: short-circuit fast path, non-element children handling, slot order independence, single-slot config edge case
3. All 303 dependent component tests pass (ActionList, ActionMenu, NavList, FormControl, TreeView, PageLayout)

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github

[changeset-bot]

🦋 Changeset detected

Latest commit: ef3c4aa

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

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.

[Copilot]

Pull request overview

This PR optimizes the useSlots hook in @primer/react, which is used widely across components to extract “slot” children while remaining SSR-compatible. The changes focus on reducing per-render allocations and enabling faster matching in common cases.

Changes:

• Replaces higher-order utilities (mapValues, findIndex) with for loops to reduce allocations and overhead.
• Adds a “all slots filled” short-circuit path to avoid further matching work.
• Adds/extends unit tests for additional slot behaviors and introduces a changeset for a patch release.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
packages/react/src/hooks/useSlots.ts	Refactors slot initialization/matching logic and adds a short-circuit path + helper functions.
packages/react/src/hooks/tests/useSlots.test.tsx	Adds tests for short-circuit behavior, non-element children handling, and slot order independence.
.changeset/perf-use-slots-short-circuit.md	Declares a patch release entry for the perf change.

[Copilot]

In production builds (DEV is false), the short-circuit path pushes all subsequent elements into rest without checking whether they match a slot. This changes behavior vs the previous implementation: duplicate slot children that appear after all slots are filled will now be treated as rest (and can be rendered), even though the warning message/semantics say duplicates are ignored (“Only the first will be rendered.”). To preserve prior behavior, avoid adding children that match any slot to rest even in the short-circuit path (you can still keep it cheap by first checking child.type against a precomputed set of configured component matchers before running the full matcher/testFn logic).

[Copilot]

warnIfDuplicate takes keys: Array<string | number | symbol>, but keys is produced via Object.keys(config) and cast to Array<keyof Config> (effectively strings). Tightening this helper signature to Array<keyof Config> (or string[]) would better reflect reality and avoid type widening that can mask mistakes.

Suggested change

	keys: Array<string | number | symbol>,
	keys: string[],

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/14138

[primer-integration]

Integration test results from github/github-ui:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!