Android pointer number remains 2 when lifting one finger during pinch gesture

[znikap]

Description

When performing a pinch gesture on Android, if one finger is lifted, the numberOfPointers property incorrectly remains at 2. This causes the focal point coordinates (focalX and focalY) to shift drastically, disrupting the pinch gesture handling.

You can view the experience here (please ignore the offsets it's a quick example):

Screen_Recording_20250305_183544_gesture-pinch-bug-android.mp4

This only happens on android, the experience of pinching is always creating the "jumping" effect.
Any help would be appreciated :3

Steps to reproduce

1. Start a pinch gesture on an Android device with two fingers.
2. Lift one finger while continuing to pinch the remaining finger.
3. Observe the values of numberOfPointers, focalX, and focalY during the onChange or onUpdate event.

Snack or a link to a repository

https://snack.expo.dev/@znikap/bug-android-on-pinch

Gesture Handler version

2.20.2, 2.24.0

UPD: Is not happening on 2.16.2

React Native version

0.74.5

Platforms

Android

JavaScript runtime

None

Workflow

Expo SDK 51

Architecture

None

Build type

None

Device

Real device

Device model

No response

Acknowledgements

Yes

[znikap]

Hey! Found out that this behavior is not happening on version 2.16.2 (works well)

[ldalzottomp]

I am upping this issue, that sounds huge. I am experiencing the same. Thanks for providing the version where the gesture works as intended.

[ldalzottomp]

A workaround for this issue would be to manually fetch the number of touches from the .onTouchesDown() and .onTouchesUp events instead.

[znikap]

@ldalzottomp Hey thanks! I solved it by tracking the number of pointers with the onTouchesDown and onTouchesUp callbacks! But still, it would be great if the devs could take a look, since this doesn't seem like expected behavior.

[haxonadora]

Add this to pinch gesture

  .onTouchesUp((e, state) => {
    if (e.numberOfTouches !== 2) {
      state.end();
    }
  })
  .onTouchesDown((e, state) => {
    if (e.numberOfTouches === 2) {
      state.begin();
    }
  })

[sorinc03]

Opened #4215 with a focused Android pinch lifecycle fix.

The issue appears to come from the Android pinch handler ignoring ScaleGestureDetector.onScaleEnd and only finishing on final ACTION_UP. That leaves the handler active for the ACTION_POINTER_UP frame when one of the two pinch fingers is lifted, so JS can receive an active update with stale numberOfPointers / focal data.

The patch ends an active pinch, or fails a begun-but-not-active pinch, when a tracked pointer lift leaves fewer than two pointers.