async _currentOutlineItem() {
    if (!this._isPagesLoaded) {
      throw new Error("_currentOutlineItem: All pages have not been loaded.");
    }
    if (!this._outline || !this._pdfDocument) {
      return;
    }

    const pageNumberToDestHash = await this._getPageNumberToDestHash(
      this._pdfDocument
    );
    if (!pageNumberToDestHash) {
      return;
    }
    this._updateCurrentTreeItem(/* treeItem = */ null);

    if (this._sidebarView !== SidebarView.OUTLINE) {
      return; // The outline view is no longer visible, hence do nothing.
    }
    // When there is no destination on the current page, always check the
    // previous ones in (reverse) order.
    for (let i = this._currentPageNumber; i > 0; i--) {
      const destHash = pageNumberToDestHash.get(i);
      if (!destHash) {
        continue;
      }
      const linkElement = this.container.querySelector(`a[href="${destHash}"]`);
      if (!linkElement) {
        continue;
      }
      this._scrollToCurrentTreeItem(linkElement.parentNode);
      break;
    }
  }
Find/highlight the current outline item, corresponding to the active page.
@private