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.
PDFViewerApplication.pdfThumbnailViewer.pagesRotation = evt.pagesRotation;
  PDFViewerApplication.forceRendering();
  
  PDFViewerApplication.pdfViewer.currentPageNumber = evt.pageNumber;
}
function webViewerPageChanging({ pageNumber, pageLabel }) {
  PDFViewerApplication.toolbar.setPageNumber(pageNumber, pageLabel);
  PDFViewerApplication.secondaryToolbar.setPageNumber(pageNumber);
  if (PDFViewerApplication.pdfSidebar.isThumbnailViewVisible) {
    PDFViewerApplication.pdfThumbnailViewer.scrollThumbnailIntoView(pageNumber);
  }
}
function webViewerVisibilityChange(evt) {
  if (document.visibilityState === "visible") {
    
    setZoomDisabledTimeout();
  }
}
let zoomDisabledTimeout = null;
function setZoomDisabledTimeout() {
  if (zoomDisabledTimeout) {
    clearTimeout(zoomDisabledTimeout);
  }
  zoomDisabledTimeout = setTimeout(function () {
    zoomDisabledTimeout = null;
  }, WHEEL_ZOOM_DISABLED_TIMEOUT);
}
function webViewerWheel(evt) {