[useResponderEvents] A move onto a sibling of the responder never asks their parent #2916

Is there an existing issue for this?

• I have searched the existing issues

Describe the issue

A parent PanResponder is never asked to take a mouse drag that goes from one child responder onto a sibling child responder. A drag that stays on the first child does ask it.

It only happens when the parent is the outermost view with responder props.

Expected behavior

The parent is asked, as when the drag stays on the first child.

Steps to reproduce

react-native-web 0.21.4, Chrome.

<View {...panResponder.panHandlers}>
  <View onStartShouldSetResponder={() => true} /> {/* A */}
  <View onStartShouldSetResponder={() => true} /> {/* B */}
</View>

Press on A, drag onto B: onMoveShouldSetPanResponder is never called.

Test case

https://github.com/Pduhard/repro-rnw-responder-lca

Additional comments

getLowestCommonAncestor (useResponderEvents/utils.js) reads the first pair twice (pathA[indexA++]), so it never compares the last one. Here the last one is the only common ancestor, so it returns null and nobody is asked:

getLowestCommonAncestor([1, 0], [2, 0]) // null, expected 0

Fix in #2917.