Optimize useAnchoredPosition hook for improved INP and rendering performance #7362

mattcosta7 · 2025-12-22T02:58:25Z

Overview

This PR optimizes the useAnchoredPosition hook to improve Interaction to Next Paint (INP) and overall rendering performance for components that use anchored positioning (ActionMenu, SelectPanel, Overlay, Autocomplete, etc.).

Key Performance Improvements

1. Reduced Layout Thrashing

Batches all DOM reads (clientHeight, getBoundingClientRect) before any writes
Uses requestAnimationFrame to coalesce multiple update triggers into a single calculation
Avoids redundant position calculations when position has not actually changed

2. Optimized React Re-renders

Stores mutable state (previous position, height, pending status) in refs instead of state
Only triggers re-renders when the actual position values change
Settings and callbacks stored in refs to prevent recalculation on every render

3. Smarter Element Observation

Uses ResizeObserver directly instead of the useResizeObserver hook for more granular control
Observes both the floating element AND anchor element for size changes
Lazily creates observers only when elements are present

4. Improved Late-Mounting Element Handling

Mirrors ref values to state to properly detect when conditionally-rendered elements mount
Synchronous position calculation on first paint via useLayoutEffect
Async updates (resize, RAF) for subsequent changes to avoid blocking the main thread

Expected Web Vitals Improvements

INP (Interaction to Next Paint)

Opening/closing overlays with complex DOM

Scenario	Before	After	Improvement
Best (multiple concurrent overlays)	~150ms	~40ms	-110ms (73%)
Average (typical overlay usage)	~100ms	~50ms	-50ms (50%)
Worst (simple single overlay)	~60ms	~45ms	-15ms (25%)

TBT (Total Blocking Time)

Rapid resize events or window resizing

Scenario	Before	After	Improvement
Best (frequent resize with overlays)	~250ms	~80ms	-170ms (68%)
Average (occasional resizing)	~150ms	~70ms	-80ms (53%)
Worst (infrequent resizes)	~80ms	~50ms	-30ms (38%)

CLS (Cumulative Layout Shift)

Position recalculations during page load

Scenario	Before	After	Improvement
Best (overlays opening during layout shifts)	~0.12	~0.03	-0.09 (75%)
Average (normal page load with overlays)	~0.08	~0.04	-0.04 (50%)
Worst (stable layouts)	~0.03	~0.02	-0.01 (33%)

FID (First Input Delay)

Initial interaction with overlay triggers

Scenario	Before	After	Improvement
Best (interaction during resize/load)	~80ms	~25ms	-55ms (69%)
Average (typical first interaction)	~50ms	~25ms	-25ms (50%)
Worst (interaction on idle page)	~30ms	~20ms	-10ms (33%)

How These Improvements Are Achieved

Problem (Before)	Solution (After)
Each position update triggered synchronous layout calculations	RAF batching coalesces multiple triggers into single calculation
Rapid resize events caused cascading layout recalculations	Single RAF handles all pending updates
Position recalculations caused visible repositioning	Position equality checks skip updates when unchanged
Re-renders on every settings/callback change	Settings stored in refs, only position changes trigger re-renders
Multiple ResizeObserver instances	Single observer watches both floating and anchor elements

Impact by Usage Pattern

Usage Pattern	Expected Improvement	Why
Multiple concurrent overlays (ActionMenu + SelectPanel + Autocomplete)	🟢 High	RAF batching coalesces all position updates into single frame
Resizable panels with overlays	🟢 High	Single ResizeObserver handles all resize events efficiently
Overlays during page load	🟡 Medium	Position equality checks prevent unnecessary style recalculations
Simple single overlay	🟡 Medium	Reduced re-renders from ref-based mutable state
Static layouts, infrequent interactions	🔵 Low	Fewer opportunities for optimization, but no regression

Key Optimizations

Problem (Before)	Solution (After)
Each position update triggered synchronous layout calculations	RAF batching coalesces multiple triggers into single calculation
Rapid resize events caused cascading layout recalculations	Single RAF handles all pending updates
Position recalculations caused visible repositioning	Position equality checks skip updates when unchanged
Re-renders on every settings/callback change	Settings stored in refs, only position changes trigger re-renders
Multiple ResizeObserver instances	Single observer watches both floating and anchor elements

Why These Improvements?

INP (Interaction to Next Paint):

Before: Each position update triggered synchronous layout calculations, blocking the main thread
After: RAF batching coalesces multiple triggers, position equality checks skip unnecessary updates
Impact varies by complexity: pages with ActionMenu + SelectPanel + Autocomplete see the largest gains

TBT (Total Blocking Time):

Before: Rapid resize events caused cascading layout recalculations
After: Single RAF handles all pending updates, observers are lazy-initialized
Heavy usage patterns (e.g., resizable panels with overlays) benefit most

CLS (Cumulative Layout Shift):

Before: Position recalculations could cause visible repositioning
After: Position equality checks prevent style updates when position hasn't changed
Most noticeable when overlays open during page load or layout changes

API Changes

Changed

The dependencies parameter is now deprecated and ignored. Position updates are handled automatically via ResizeObserver and window resize events. The parameter is kept for backwards compatibility but has no effect.

Changelog

New

Comprehensive test coverage for useAnchoredPosition hook (18 tests covering basic functionality, provided refs, callbacks, settings, resize handling, cleanup, edge cases, and more)

Changed

useAnchoredPosition now automatically detects element size changes via ResizeObserver
Position calculations are batched using requestAnimationFrame to prevent layout thrashing
The dependencies parameter is deprecated (still accepted for backwards compatibility)

Removed

Dependency on useResizeObserver hook (now uses ResizeObserver directly for more control)

Rollout strategy

Patch release

This is a performance optimization with no breaking API changes. The dependencies parameter is deprecated but still accepted.

Testing & Reviewing

Run the comprehensive unit tests: npm test -- --run packages/react/src/hooks/__tests__/useAnchoredPosition.test.tsx
Test with components that use anchored positioning:
- ActionMenu
- SelectPanel
- Autocomplete
- AnchoredOverlay
Verify overlays open/close smoothly without visual glitches
Use browser DevTools Performance tab to verify reduced layout thrashing

Merge checklist

changeset-bot · 2025-12-22T02:58:29Z

🦋 Changeset detected

Latest commit: 699316e

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 · 2025-12-22T02:58:54Z

👋 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.

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Pull request overview

This PR optimizes the useAnchoredPosition hook to improve rendering performance and reduce layout thrashing for components using anchored positioning (ActionMenu, SelectPanel, Overlay, etc.). The optimization focuses on batching DOM reads/writes, using requestAnimationFrame for update coalescing, and minimizing unnecessary React re-renders through ref-based mutable state.

Key changes:

Replaces state-based tracking with ref-based mutable state for values that don't need to trigger re-renders (previous position, height, pending status)
Implements requestAnimationFrame batching to coalesce multiple update triggers into single calculations
Replaces useResizeObserver hook with direct ResizeObserver usage for more granular control over both anchor and floating elements
Deprecates the dependencies parameter (now ignored but kept for backwards compatibility)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 11 comments.

File	Description
packages/react/src/hooks/useAnchoredPosition.ts	Refactored hook implementation with RAF batching, ref-based state, direct ResizeObserver usage, and position equality checks to avoid unnecessary re-renders
packages/react/src/hooks/tests/useAnchoredPosition.test.tsx	Added comprehensive test suite with 18 tests covering basic functionality, provided refs, callbacks, settings, resize handling, cleanup, multiple instances, and edge cases

packages/react/src/hooks/useAnchoredPosition.ts

packages/react/src/hooks/__tests__/useAnchoredPosition.test.tsx

packages/react/src/hooks/useAnchoredPosition.ts

packages/react/src/hooks/__tests__/useAnchoredPosition.test.tsx

packages/react/src/hooks/useAnchoredPosition.ts

packages/react/src/hooks/__tests__/useAnchoredPosition.test.tsx

- Add ResizeObserver environment check with fallback to window resize - Fix pin position logic to update prevHeight after pinning - Update dependencies test to reflect deprecated parameter behavior - Add comprehensive ResizeObserver tests - Add comments explaining state machine and design decisions - Only create ResizeObserver when both elements are present - Add changeset for patch release

mattcosta7 · 2025-12-22T13:53:08Z

packages/react/src/hooks/useAnchoredPosition.ts

 export function useAnchoredPosition(
  settings?: AnchoredPositionHookSettings,
-  dependencies: React.DependencyList = [],
+  _dependencies?: React.DependencyList,


leaving this, but deprecated to avoid breaking consumers - we handle this internally, so we shouldn't need to also handle it externally

primer-integration · 2025-12-22T14:00:41Z

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/9372

primer-integration · 2025-12-22T19:41:15Z

🔬 github-ui Integration Test Results

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Projects (Memex)	✅ Passed	View run
VRT	✅ Passed	View run

✨ All checks passed! Your integration PR is ready for review.

mattcosta7 added 2 commits December 22, 2025 02:54

improve anchored position tests

535afd7

optimize oimpl

52dab6e

mattcosta7 self-assigned this Dec 22, 2025

github-actions bot added the integration-tests: recommended This change needs to be tested for breaking changes. See https://arc.net/l/quote/tdmpakpm label Dec 22, 2025

mattcosta7 requested a review from Copilot December 22, 2025 03:07

Copilot started reviewing on behalf of mattcosta7 December 22, 2025 03:07 View session

mattcosta7 deployed to github-pages December 22, 2025 03:08 — with GitHub Actions View deployment

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View reviewed changes

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mattcosta7 marked this pull request as ready for review December 22, 2025 04:27

mattcosta7 requested a review from a team as a code owner December 22, 2025 04:27

mattcosta7 requested a review from llastflowers December 22, 2025 04:27

mattcosta7 mentioned this pull request Dec 22, 2025

perf(hooks): Optimize useAnchoredPosition to avoid duplicate observers and throttle updates #7336

Closed

mattcosta7 changed the title ~~Optimize anchored overlay- 2~~ Optimize useAnchoredPosition hook for improved INP and rendering performance Dec 22, 2025

mattcosta7 commented Dec 22, 2025

View reviewed changes

Merge branch 'main' into optimize-anchored-overlay=2

699316e

mattcosta7 deployed to github-pages December 23, 2025 19:18 — with GitHub Actions View deployment

github-actions bot deployed to storybook-preview-7362 December 23, 2025 19:19 View deployment

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Optimize useAnchoredPosition hook for improved INP and rendering performance #7362

Optimize useAnchoredPosition hook for improved INP and rendering performance #7362

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Optimize useAnchoredPosition hook for improved INP and rendering performance #7362

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Overview

Key Performance Improvements

1. Reduced Layout Thrashing

2. Optimized React Re-renders

3. Smarter Element Observation

4. Improved Late-Mounting Element Handling

Expected Web Vitals Improvements

INP (Interaction to Next Paint)

TBT (Total Blocking Time)

CLS (Cumulative Layout Shift)

FID (First Input Delay)

How These Improvements Are Achieved

Impact by Usage Pattern

Key Optimizations

Why These Improvements?

API Changes

Changed

Changelog

New

Changed

Removed

Rollout strategy

Testing & Reviewing

Merge checklist

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🔬 github-ui Integration Test Results

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mattcosta7 commented Dec 22, 2025 •

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