This is a tricky one since pencil and finger triggers exactly the same callbacks. It turns out that when pencil is involved after finger interaction, the platform view's "forwarding" gesture recognizer is stuck at failed state. This seems to be an iOS bug, because according to the API doc, it should be reset back to "possible" state:

No action message is sent and the gesture recognizer is reset to UIGestureRecognizerStatePossible.

However, when iPad pencil is involved, the state is not reset. I tried to KVO the state property, and wasn't able to capture the change. This means the state change very likely happened internally within the recognizer via the backing ivar of the state property.

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Fixes flutter/flutter#136244

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