TouchGestureTracker follows two fingers and reports the pan, scale and
rotation since the previous update around their midpoint. The Android
window tracks every finger, sends a gesture while two or more touch
the screen and releases the pointer the first finger was holding, so
one finger keeps acting as a pointer. Gestures travel through the input
coalescer, which merges consecutive updates, to a new Gesture UI event
delivered to the widget under the fingers where the gesture began.
Gesture events reach the UI before pointer events from the same frame.
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