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.requestPresentationMode();
}
function webViewerPrint() {
  PDFViewerApplication.triggerPrinting();
}
function webViewerDownload() {
  PDFViewerApplication.downloadOrSave({ sourceEventType: "download" });
}
function webViewerSave() {
  PDFViewerApplication.downloadOrSave({ sourceEventType: "save" });
}
function webViewerFirstPage() {
  if (PDFViewerApplication.pdfDocument) {
    PDFViewerApplication.page = 1;
  }
}
function webViewerLastPage() {
  if (PDFViewerApplication.pdfDocument) {
    PDFViewerApplication.page = PDFViewerApplication.pagesCount;
  }
}
function webViewerNextPage() {
  PDFViewerApplication.pdfViewer.nextPage();
}
function webViewerPreviousPage() {
  PDFViewerApplication.pdfViewer.previousPage();
}
function webViewerZoomIn() {