…d outside gestures

Summary:

There's a bug in ScrollView that occasionally causes unexpected fling to happen.
For example:
Horizontal ScrollView created in rtl layout with overScrollMode='never' (this prop makes it easy to reproduce this bug).

Let's assume some data loaded immediately, some after a moment. This causes list size to change, therefore onLayoutChange is called in ReactHorizontalScrollView, which updates scroll offset accordingly:

adjustPositionForContentChangeRTL(left, right, oldLeft, oldRight);

This can e.g. change scroll from offset 5000 to 10000 (initial position).

Due to the way OnScrollDispatchHelper tracks scroll velocity now, this scroll change will be tracked as if it was made during a gesture, and thus its "velocity" is saved.

With saved velocity, making a "zero pixel gesture", e.g. trying to overscroll after the scrollview start, will cause fling to happen, because velocity is non-zero:

public void fling(int velocityX) {
(...)
final int correctedVelocityX =
        Build.VERSION.SDK_INT == Build.VERSION_CODES.P
            ? (int) (Math.abs(velocityX) * Math.signum(mOnScrollDispatchHelper.getXFlingVelocity()))
            : velocityX;

Note: This will happen after a manual .scrollTo() call, too.

Changelog:

Test Plan:

Without fix trying to overscroll list start, unexpected fling happens in the opposite direction:

Fixed, no fling happens when trying to overscroll: