constructor(draggable) {
    super(draggable);

    /**
     * Scrollable options
     * @property {Object} options
     * @property {Number} options.speed
     * @property {Number} options.sensitivity
     * @property {HTMLElement[]} options.scrollableElements
     * @type {Object}
     */
    this.options = {
      ...defaultOptions,
      ...this.getOptions(),
    };

    /**
     * Keeps current mouse position
     * @property {Object} currentMousePosition
     * @property {Number} currentMousePosition.clientX
     * @property {Number} currentMousePosition.clientY
     * @type {Object|null}
     */
    this.currentMousePosition = null;

    /**
     * Scroll animation frame
     * @property scrollAnimationFrame
     * @type {Number|null}
     */
    this.scrollAnimationFrame = null;

    /**
     * Closest scrollable element
     * @property scrollableElement
     * @type {HTMLElement|null}
     */
    this.scrollableElement = null;

    /**
     * Animation frame looking for the closest scrollable element
     * @property findScrollableElementFrame
     * @type {Number|null}
     */
    this.findScrollableElementFrame = null;

    this[onDragStart] = this[onDragStart].bind(this);
    this[onDragMove] = this[onDragMove].bind(this);
    this[onDragStop] = this[onDragStop].bind(this);
    this[scroll] = this[scroll].bind(this);
  }
Scrollable constructor.
@constructs Scrollable
@param {Draggable} draggable - Draggable instance