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 eventHandler = handler.bind(null, WaitOnType.EVENT);
    if (target instanceof EventBus) {
      target._on(name, eventHandler);
    } else {
      target.addEventListener(name, eventHandler);
    }
    const timeoutHandler = handler.bind(null, WaitOnType.TIMEOUT);
    const timeout = setTimeout(timeoutHandler, delay);
  });
}
/**
 * Promise that is resolved when DOM window becomes visible.
 */
const animationStarted = new Promise(function (resolve) {
  if (
    typeof PDFJSDev !== "undefined" &&
    PDFJSDev.test("LIB && TESTING") &&
    typeof window === "undefined"
  ) {
    
    
    setTimeout(resolve, 20);
    return;
  }
  window.requestAnimationFrame(resolve);
});
/**
 * NOTE: Only used to support various PDF viewer tests in `mozilla-central`.
 */
function dispatchDOMEvent(eventName, args = null) {