handleDraggerStart_(event) {
    if (!this.dragging_ && event.target === this.element.firstElementChild) {
      const element = /** @type {HTMLElement} */ (
        this.element.firstElementChild
      );
      this.getMap().getView().beginInteraction();
      this.startX_ = event.clientX - parseFloat(element.style.left);
      this.startY_ = event.clientY - parseFloat(element.style.top);
      this.dragging_ = true;

      if (this.dragListenerKeys_.length === 0) {
        const drag = this.handleDraggerDrag_;
        const end = this.handleDraggerEnd_;
        const doc = this.getMap().getOwnerDocument();
        this.dragListenerKeys_.push(
          listen(doc, PointerEventType.POINTERMOVE, drag, this),
          listen(doc, PointerEventType.POINTERUP, end, this),
        );
      }
    }
  }
Handle dragger start events.
@param {PointerEvent} event The drag event.
@private