STR:
  - Go to https://mozilla.github.io/pdf.js/web/viewer.html
  - Click on "Open File" and open a pdf

The `Download` button and the `Current view` one just disappear.
I can reproduce this bug with Firefox nightly, Chrome stable and Epiphany on debian.
this.externalServices.reportTelemetry({
      type: "unsupportedFeature",
      featureId,
    });
    
    
    if (this._fellback) {
      return;
    }
    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) {