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 [x1, y1, x2, y2] = view;
  
  const changeOrientation = rotate % 180 !== 0;
  const width = ((x2 - x1) / 72) * userUnit;
  const height = ((y2 - y1) / 72) * userUnit;
  return {
    width: changeOrientation ? height : width,
    height: changeOrientation ? width : height,
  };
}
/**
 * Helper function for getVisibleElements.
 *
 * @param {number} index - initial guess at the first visible element
 * @param {Array} views - array of pages, into which `index` is an index
 * @param {number} top - the top of the scroll pane
 * @returns {number} less than or equal to `index` that is definitely at or
 *   before the first visible element in `views`, but not by too much. (Usually,
 *   this will be the first element in the first partially visible row in
 *   `views`, although sometimes it goes back one row further.)
 */
function backtrackBeforeAllVisibleElements(index, views, top) {