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 (PDFViewerApplication.overlayManager.active) {
    return;
  }
  let handled = false,
    ensureViewerFocused = false;
  const cmd =
    (evt.ctrlKey ? 1 : 0) |
    (evt.altKey ? 2 : 0) |
    (evt.shiftKey ? 4 : 0) |
    (evt.metaKey ? 8 : 0);
  const pdfViewer = PDFViewerApplication.pdfViewer;
  const isViewerInPresentationMode =
    pdfViewer && pdfViewer.isInPresentationMode;
  
  
  if (cmd === 1 || cmd === 8 || cmd === 5 || cmd === 12) {
    
    switch (evt.keyCode) {
      case 70: 
        if (!PDFViewerApplication.supportsIntegratedFind && !evt.shiftKey) {
          PDFViewerApplication.findBar.open();
          handled = true;
        }
        break;
      case 71: 
        if (!PDFViewerApplication.supportsIntegratedFind) {
          const findState = PDFViewerApplication.findController.state;