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 (Math.abs(pc) > 0.001) {
        return -pc;
      }
      return a.id - b.id; 
    });
  }
  return { first, last, views: visible };
}
/**
 * Event handler to suppress context menu.
 */
function noContextMenuHandler(evt) {
  evt.preventDefault();
}
function isDataSchema(url) {
  let i = 0;
  const ii = url.length;
  while (i < ii && url[i].trim() === "") {
    i++;
  }
  return url.substring(i, i + 5).toLowerCase() === "data:";
}
/**
 * Returns the filename or guessed filename from the url (see issue 3455).
 * @param {string} url - The original PDF location.
 * @param {string} defaultFilename - The value returned if the filename is
 *   unknown, or the protocol is unsupported.
 * @returns {string} Guessed PDF filename.
 */
function getPDFFileNameFromURL(url, defaultFilename = "document.pdf") {