function onMousewheel( event ) {
	if ( event.defaultPrevented ) {
		return;
	}

	const target = event.target;
	let overflowing = overflowingAncestor( target );

	// use default if there's no overflowing element
	if ( !overflowing ) {
		return true;
	}

	event.preventDefault();

	let deltaX = event.wheelDeltaX || 0;
	let deltaY = event.wheelDeltaY || 0;

	// use wheelDelta if deltaX/Y is not available
	if ( !deltaX && !deltaY ) {
		deltaY = event.wheelDelta || 0;
	}

	// scale by step size
	// delta is 120 most of the time
	// synaptics seems to send 1 sometimes
	if ( Math.abs( deltaX ) > 1.2 ) {
		deltaX *= options.stepSize / 120;
	}
	if ( Math.abs( deltaY ) > 1.2 ) {
		deltaY *= options.stepSize / 120;
	}

	scrollArray( overflowing, -deltaX, -deltaY );
}
Mouse wheel handler.
@param {MouseEvent} event