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 view;
  switch (mode) {
    case "thumbs":
      view = SidebarView.THUMBS;
      break;
    case "bookmarks":
    case "outline": 
      view = SidebarView.OUTLINE;
      break;
    case "attachments": 
      view = SidebarView.ATTACHMENTS;
      break;
    case "layers": 
      view = SidebarView.LAYERS;
      break;
    case "none":
      view = SidebarView.NONE;
      break;
    default:
      console.error('Invalid "pagemode" hash parameter: ' + mode);
      return;
  }
  PDFViewerApplication.pdfSidebar.switchView(view,  true);
}
function webViewerNamedAction(evt) {