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.
if (this.isViewerEmbedded) {
      
      return;
    }
    document.title = title;
  },
  get _docFilename() {
    
    
    return this._contentDispositionFilename || getPdfFilenameFromUrl(this.url);
  },
  /**
   * @private
   */
  _cancelIdleCallbacks() {
    if (!this._idleCallbacks.size) {
      return;
    }
    for (const callback of this._idleCallbacks) {
      window.cancelIdleCallback(callback);
    }
    this._idleCallbacks.clear();
  },
  /**
   * Closes opened PDF document.
   * @returns {Promise} - Returns the promise, which is resolved when all
   *                      destruction is completed.
   */
  async close() {