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 (
      viewBottom <= top ||
      currentHeight >= bottom ||
      viewRight <= left ||
      currentWidth >= right
    ) {
      continue;
    }
    const hiddenHeight =
      Math.max(0, top - currentHeight) + Math.max(0, viewBottom - bottom);
    const hiddenWidth =
      Math.max(0, left - currentWidth) + Math.max(0, viewRight - right);
    const percent =
      (((viewHeight - hiddenHeight) * (viewWidth - hiddenWidth) * 100) /
        viewHeight /
        viewWidth) |
      0;
    visible.push({
      id: view.id,
      x: currentWidth,
      y: currentHeight,
      view,
      percent,
    });
  }
  const first = visible[0],
    last = visible[visible.length - 1];
  if (sortByVisibility) {
    visible.sort(function (a, b) {
      const pc = a.percent - b.percent;