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
DARK: 2,
};
const KNOWN_VERSIONS = [
  "1.0",
  "1.1",
  "1.2",
  "1.3",
  "1.4",
  "1.5",
  "1.6",
  "1.7",
  "1.8",
  "1.9",
  "2.0",
  "2.1",
  "2.2",
  "2.3",
];
const KNOWN_GENERATORS = [
  "acrobat distiller",
  "acrobat pdfwriter",
  "adobe livecycle",
  "adobe pdf library",
  "adobe photoshop",
  "ghostscript",
  "tcpdf",
  "cairo",
  "dvipdfm",
  "dvips",
  "pdftex",