This problem applies to all the documents that I have ever opened.

Configuration:
- Web browser and its version: Google Chrome 83.0.4103.61 , also Firefox 76.0.1
- Operating system and its version: Win 7 x64
- PDF.js version: 2.3.164 version of extension
- Is a browser extension: yes

Steps to reproduce the problem:
1. Select any "Spreads" mode of view. Or "No spreads", but wrapped scrolling mode - doesn't matter. 
2. Select "Page Fit" zoom mode. We should get several pages per screen
3. Try to scroll document by any of keys: Page Down, n, j, Space Bar

I expect to see the next group of pages (for example, if there are 5 pages on the screen, I expect to see the next 5 pages)

What went wrong? Scrolling goes one page at a time, so if I have 5 pages on the screen, nothing happens when I pressed Page Down 5 times . For the 6th time I finally see the next group of pages.

Just for example: Adobe Reader scrolls properly in two-page mode, 2 pages at once. 

P.S. Sorry for my English, I hope I correctly stated the essence of the problem
if (typeof Stats === "undefined" || !Stats.enabled) {
    return;
  }
  const pageView = PDFViewerApplication.pdfViewer.getPageView(
     pageNumber - 1
  );
  const pageStats = pageView && pageView.pdfPage && pageView.pdfPage.stats;
  if (!pageStats) {
    return;
  }
  Stats.add(pageNumber, pageStats);
}
function webViewerInitialized() {
  const appConfig = PDFViewerApplication.appConfig;
  let file;
  if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
    const queryString = document.location.search.substring(1);
    const params = parseQueryString(queryString);
    file = "file" in params ? params.file : AppOptions.get("defaultUrl");
    validateFileURL(file);
  } else if (PDFJSDev.test("MOZCENTRAL")) {
    file = window.location.href;
  } else if (PDFJSDev.test("CHROME")) {
    file = AppOptions.get("defaultUrl");
  }
  if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
    const fileInput = document.createElement("input");
    fileInput.id = appConfig.openFileInputName;
    fileInput.className = "fileInput";