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 (
      turnPage !== 0 &&
      (!turnOnlyIfPageFit || pdfViewer.currentScaleValue === "page-fit")
    ) {
      if (turnPage > 0) {
        if (PDFViewerApplication.page < PDFViewerApplication.pagesCount) {
          PDFViewerApplication.page++;
        }
      } else {
        if (PDFViewerApplication.page > 1) {
          PDFViewerApplication.page--;
        }
      }
      handled = true;
    }
  }
  
  if (cmd === 4) {
    switch (evt.keyCode) {
      case 13: 
      case 32: 
        if (
          !isViewerInPresentationMode &&
          pdfViewer.currentScaleValue !== "page-fit"
        ) {
          break;
        }
        if (PDFViewerApplication.page > 1) {
          PDFViewerApplication.page--;