onMouseWheel_(event) {
    if (this.ws_.isDisabled()) {
      return;
    }

    if (this.ws_.isMoving || this.timeout_) {
      event.preventDefault();
      return;
    }

    // Firefox uses lines instead of pixels for delta
    const linesToPx = event.deltaMode * this.ws_.options.minWheelDelta;
    const {deltaY: wheelDeltaY, deltaX: wheelDeltaX} = event;
    const isVertical = this.ws_.isVertical;
    const isHorizontalMovement = Math.abs(wheelDeltaX) > Math.abs(wheelDeltaY);
    this.isGoingUp_ = wheelDeltaY < 0;
    this.isGoingLeft_ = wheelDeltaX < 0;


    // If we're mainly moving horizontally, prevent default
    if (isHorizontalMovement) {
      if (!isVertical) {
        event.preventDefault();
      } else {
        // If we're moving horizontally but this is vertical, return to avoid
        // unwanted navigation.
        return;
      }
    }

    if (Math.abs(wheelDeltaY + linesToPx) >= this.ws_.options.minWheelDelta ||
        Math.abs(wheelDeltaX + linesToPx) >= this.ws_.options.minWheelDelta) {
      if ((isHorizontalMovement && this.isGoingLeft_) ||
          (!isHorizontalMovement && this.isGoingUp_)) {
        this.ws_.goPrev();
      } else {
        this.ws_.goNext();
      }

      event.preventDefault();
    }
  }
Reacts to the wheel event. Detects whether is going up or down and decides
if it needs to move the slide based on the amount of delta.
@param {WheelEvent} event The Wheel Event.
@private