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.
eventBus._off("zoomin", webViewerZoomIn);
    eventBus._off("zoomout", webViewerZoomOut);
    eventBus._off("zoomreset", webViewerZoomReset);
    eventBus._off("pagenumberchanged", webViewerPageNumberChanged);
    eventBus._off("scalechanged", webViewerScaleChanged);
    eventBus._off("rotatecw", webViewerRotateCw);
    eventBus._off("rotateccw", webViewerRotateCcw);
    eventBus._off("optionalcontentconfig", webViewerOptionalContentConfig);
    eventBus._off("switchscrollmode", webViewerSwitchScrollMode);
    eventBus._off("scrollmodechanged", webViewerScrollModeChanged);
    eventBus._off("switchspreadmode", webViewerSwitchSpreadMode);
    eventBus._off("spreadmodechanged", webViewerSpreadModeChanged);
    eventBus._off("documentproperties", webViewerDocumentProperties);
    eventBus._off("find", webViewerFind);
    eventBus._off("findfromurlhash", webViewerFindFromUrlHash);
    eventBus._off("updatefindmatchescount", webViewerUpdateFindMatchesCount);
    eventBus._off("updatefindcontrolstate", webViewerUpdateFindControlState);
    if (_boundEvents.reportPageStatsPDFBug) {
      eventBus._off("pagerendered", _boundEvents.reportPageStatsPDFBug);
      eventBus._off("pagechanging", _boundEvents.reportPageStatsPDFBug);
      _boundEvents.reportPageStatsPDFBug = null;
    }
    if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
      eventBus._off("fileinputchange", webViewerFileInputChange);
      eventBus._off("openfile", webViewerOpenFile);
    }
    _boundEvents.beforePrint = null;
    _boundEvents.afterPrint = null;
  },
  unbindWindowEvents() {