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
const result = [];
      let lineTop = 0,
        id = 0;
      for (const line of lines) {
        const lineHeight = line.reduce(function (maxHeight, pair) {
          return Math.max(maxHeight, pair[1]);
        }, 0);
        let offsetLeft = -BORDER_WIDTH;
        for (const [clientWidth, clientHeight] of line) {
          const offsetTop =
            lineTop + (lineHeight - clientHeight) / 2 - BORDER_WIDTH;
          const div = {
            offsetLeft,
            offsetTop,
            clientWidth,
            clientHeight,
            clientLeft: BORDER_WIDTH,
            clientTop: BORDER_WIDTH,
          };
          result.push({ id, div });
          ++id;
          offsetLeft += clientWidth + SPACING;
        }
        lineTop += lineHeight + SPACING;
      }
      return result;
    }
    
    
    function slowGetVisibleElements(scroll, pages) {