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
elt = views[index - 1].div;
    pageTop = elt.offsetTop + elt.clientTop;
  }
  
  
  
  
  
  
  
  
  for (let i = index - 2; i >= 0; --i) {
    elt = views[i].div;
    if (elt.offsetTop + elt.clientTop + elt.clientHeight <= pageTop) {
      
      
      
      break;
    }
    index = i;
  }
  return index;
}
/**
 * @typedef {Object} GetVisibleElementsParameters
 * @property {HTMLElement} scrollEl - A container that can possibly scroll.
 * @property {Array} views - Objects with a `div` property that contains an
 *   HTMLElement, which should all be descendants of `scrollEl` satisfying the