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 (
          viewLeft < scrollRight &&
          viewRight > scrollLeft &&
          viewTop < scrollBottom &&
          viewBottom > scrollTop
        ) {
          const hiddenHeight =
            Math.max(0, scrollTop - viewTop) +
            Math.max(0, viewBottom - scrollBottom);
          const hiddenWidth =
            Math.max(0, scrollLeft - viewLeft) +
            Math.max(0, viewRight - scrollRight);
          const visibleArea =
            (div.clientHeight - hiddenHeight) * (div.clientWidth - hiddenWidth);
          const percent =
            ((visibleArea * 100) / div.clientHeight / div.clientWidth) | 0;
          views.push({ id: view.id, x: viewLeft, y: viewTop, view, percent });
        }
      }
      return { first: views[0], last: views[views.length - 1], views };
    }
    
    
    
    function scrollOverDocument(pages, horizontal = false, rtl = false) {
      const size = pages.reduce(function (max, { div }) {
        return Math.max(
          max,
          horizontal
            ? Math.abs(div.offsetLeft + div.clientLeft + div.clientWidth)