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
_eventHandler: debounceScroll,
  };
  let rAF = null;
  viewAreaElement.addEventListener("scroll", debounceScroll, true);
  return state;
}
/**
 * Helper function to parse query string (e.g. ?param1=value&parm2=...).
 */
function parseQueryString(query) {
  const parts = query.split("&");
  const params = Object.create(null);
  for (let i = 0, ii = parts.length; i < ii; ++i) {
    const param = parts[i].split("=");
    const key = param[0].toLowerCase();
    const value = param.length > 1 ? param[1] : null;
    params[decodeURIComponent(key)] = decodeURIComponent(value);
  }
  return params;
}
/**
 * Use binary search to find the index of the first item in a given array which
 * passes a given condition. The items are expected to be sorted in the sense
 * that if the condition is true for one item in the array, then it is also true
 * for all following items.
 *
 * @returns {number} Index of the first array element to pass the test,
 *                   or |items.length| if no such element exists.
 */
function binarySearchFirstItem(items, condition) {