captureMouseMoves(/*object*/ event) {
    if (!this._eventMoveToken && !this._eventUpToken) {
      this._eventMoveToken = EventListener.listen(
        this._domNode,
        'mousemove',
        this._onMouseMove
      );
      this._eventUpToken = EventListener.listen(
        this._domNode,
        'mouseup',
        this._onMouseUp
      );
    }

    if (!this._isDragging) {
      this._deltaX = 0;
      this._deltaY = 0;
      this._isDragging = true;
      this._x = event.clientX;
      this._y = event.clientY;
    }
    event.preventDefault();
  }
This is to set up the listeners for listening to mouse move
and mouse up signaling the movement has ended. Please note that these
listeners are added at the document.body level. It takes in an event
in order to grab inital state.