startDragging() {
    this._mouseMoveHandler = this.mouseMoveHandler.bind(this);
    this._stopDragging = this.stopDragging.bind(this);
    document.body.addEventListener(`mousemove`, this._mouseMoveHandler);
    document.body.addEventListener(`mouseup`, this._stopDragging);
    document.body.addEventListener(`mouseleave`, this._stopDragging);
  }
Action to trigger whenever the drag handle is moved.
Pass this action through the template.

@property on-drag
@type {Function}