Since #12241 we have notifications for this when the user navigates away from the viewer, but we do not yet warn when switching documents, which means that if you call e.g. `PDFViewerApplication.open` with a new document (or directly call `BaseViewer.setDocument` when using the viewer components standalone) there won't be any warning about losing form data. This should be addressed.
this.fellback = true;
    this.externalServices
      .fallback({
        featureId,
        url: this.baseUrl,
      })
      .then(download => {
        if (!download) {
          return;
        }
        this.download({ sourceEventType: "download" });
      });
  },
  /**
   * Show the error box; used for errors affecting loading and/or parsing of
   * the entire PDF document.
   */
  _documentError(message, moreInfo = null) {
    this._unblockDocumentLoadEvent();
    this._otherError(message, moreInfo);
  },
  /**
   * Show the error box; used for errors affecting e.g. only a single page.
   *
   * @param {string} message - A message that is human readable.
   * @param {Object} [moreInfo] - Further information about the error that is
   *                              more technical.  Should have a 'message' and
   *                              optionally a 'stack' property.
   */
  _otherError(message, moreInfo = null) {