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 (typeof PDFJSDev !== "undefined" && !PDFJSDev.test("MOZCENTRAL")) {
    throw new Error("Not implemented: dispatchDOMEvent");
  }
  const details = Object.create(null);
  if (args && args.length > 0) {
    const obj = args[0];
    for (const key in obj) {
      const value = obj[key];
      if (key === "source") {
        if (value === window || value === document) {
          return; 
        }
        continue; 
      }
      details[key] = value;
    }
  }
  const event = document.createEvent("CustomEvent");
  event.initCustomEvent(eventName, true, true, details);
  document.dispatchEvent(event);
}
/**
 * Simple event bus for an application. Listeners are attached using the `on`
 * and `off` methods. To raise an event, the `dispatch` method shall be used.
 */
class EventBus {