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.
let turnPage = 0,
      turnOnlyIfPageFit = false;
    switch (evt.keyCode) {
      case 38: 
      case 33: 
        
        if (pdfViewer.isVerticalScrollbarEnabled) {
          turnOnlyIfPageFit = true;
        }
        turnPage = -1;
        break;
      case 8: 
        if (!isViewerInPresentationMode) {
          turnOnlyIfPageFit = true;
        }
        turnPage = -1;
        break;
      case 37: 
        
        if (pdfViewer.isHorizontalScrollbarEnabled) {
          turnOnlyIfPageFit = true;
        }
      
      case 75: 
      case 80: 
        turnPage = -1;
        break;
      case 27: 
        if (PDFViewerApplication.secondaryToolbar.isOpen) {
          PDFViewerApplication.secondaryToolbar.close();