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
return (
    Number.isInteger(mode) &&
    Object.values(SpreadMode).includes(mode) &&
    mode !== SpreadMode.UNKNOWN
  );
}
function isPortraitOrientation(size) {
  return size.width <= size.height;
}
const WaitOnType = {
  EVENT: "event",
  TIMEOUT: "timeout",
};
/**
 * @typedef {Object} WaitOnEventOrTimeoutParameters
 * @property {Object} target - The event target, can for example be:
 *   `window`, `document`, a DOM element, or an {EventBus} instance.
 * @property {string} name - The name of the event.
 * @property {number} delay - The delay, in milliseconds, after which the
 *   timeout occurs (if the event wasn't already dispatched).
 */
/**
 * Allows waiting for an event or a timeout, whichever occurs first.
 * Can be used to ensure that an action always occurs, even when an event
 * arrives late or not at all.
 *
 * @param {WaitOnEventOrTimeoutParameters}
 * @returns {Promise} A promise that is resolved with a {WaitOnType} value.
 */
function waitOnEventOrTimeout({ target, name, delay = 0 }) {