[Web] Fix view relative coordinates of trackpad pan events

[giaBaoJS]

Description

WheelEventManager synthesizes a moving pointer out of a stationary cursor by accumulating wheel deltas, but only the page coordinates use the accumulator:

this.wheelDelta.x += event.deltaX;      // wheelCallback

x: event.clientX + this.wheelDelta.x,   // accumulated
offsetX: event.offsetX - event.deltaX,  // one event, opposite sign

Repro: a Pan with enableTrackpadTwoFingerGesture on web, three trackpad frames of deltaY: 10 from clientY: 0, offsetY: 0. absoluteY reads 10, 20, 30 while y reads -10, -10, -10. y points the wrong way and never advances for the whole gesture. translationY, velocityY and absoluteY are fine, they come off the page coordinates.

x/y and absoluteX/absoluteY are the same point in two spaces, so they can only differ by the view's page offset. Every other producer keeps that invariant: PointerEventManager.ts:230 derives offsetX from the same clientX it writes to x, KeyboardEventManager.ts:120-129 writes the view centre into both, ScrollEventManager.ts:31-34 writes one value twice, and GestureHandlerButton.web.tsx:51-53 computes x from absoluteX. WheelEventManager.ts:35-36 is the only place where the two move apart.

The fix applies the same accumulator to both. onWheel is overridden only by PanGestureHandler, and the wheel offsetX/offsetY reaches exactly one consumer, PointerTracker.relativeCoords -> GestureHandler.transformNativeEvent():759.

Boundary: the base stays event.offsetX, which is relative to event.target rather than to the handler's view, so a wheel over a child element is still off by that child's position. PointerEventManager sidesteps that with (clientX - rect.left) / scaleX, but doing the same here costs a getBoundingClientRect per wheel event, which ScrollEventManager.mapEvent explicitly declines to pay. Left as a separate question.

Test plan

yarn workspace react-native-gesture-handler test - 179 passed, was 178.

The new PanGestureHandler trackpad coordinates case drives the real WheelEventManager off a fake view and asserts the two spaces agree for a view at the page origin. On main it reports x: -5, y: -10 against absoluteX: 15, absoluteY: 30.

ts-check, lint-js and circular-dependency-check are clean.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Advanced

Run ID: 02622e14-eabd-480a-8f3d-23e665e38c12

📥 Commits

Reviewing files that changed from the base of the PR and between 4c001cb and ad0de17.

📒 Files selected for processing (2)

• packages/react-native-gesture-handler/src/web/handlers/__tests__/PanGestureHandler.test.ts
• packages/react-native-gesture-handler/src/web/tools/WheelEventManager.ts

Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review.

---

📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Improved trackpad gesture coordinate handling on web.
  • Corrected horizontal and vertical offset calculations when wheel movement is combined with pointer movement.
  • Ensured view-relative coordinates remain consistent with page coordinates.

Walkthrough

Wheel-event coordinate mapping now uses accumulated deltas for transformed offsets. Tests add horizontal wheel input and verify accumulated view-relative and page coordinates.

Changes

Wheel coordinate mapping

Layer / File(s)	Summary
Accumulated wheel coordinate mapping packages/react-native-gesture-handler/src/web/tools/WheelEventManager.ts	mapEvent now adds accumulated wheel deltas to the raw horizontal and vertical offsets.
Trackpad coordinate regression coverage packages/react-native-gesture-handler/src/web/handlers/__tests__/PanGestureHandler.test.ts	TestPanGestureHandler exposes the transformed native event. FakeView.wheel forwards an optional horizontal delta. Tests verify accumulated trackpad coordinates on both axes.

Priority: ⬇️ Low

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: fixing view-relative coordinates for web trackpad pan events.
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.

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[Copilot]

Copilot review overview

🟢 Approval recommended

The fix is localized, consistent with existing coordinate handling, and covered by regression tests.

Review effort: Lite
Findings: None

What changed in this PR

Fixes web trackpad pan coordinates so relative x/y advance consistently with absolute coordinates.

Changes:

• Applies accumulated wheel deltas to offsetX and offsetY.
• Adds regression coverage for trackpad coordinate consistency.

File	Description
WheelEventManager.ts	Corrects accumulated relative wheel coordinates.
PanGestureHandler.test.ts	Adds trackpad coordinate regression tests.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.