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 visible = this._getVisiblePages();
    const visiblePages = visible.views,
      numVisiblePages = visiblePages.length;
    if (numVisiblePages === 0) {
      return;
    }
    const newCacheSize = Math.max(DEFAULT_CACHE_SIZE, 2 * numVisiblePages + 1);
    this._buffer.resize(newCacheSize, visiblePages);
    this.renderingQueue.renderHighestPriority(visible);
    this._updateHelper(visiblePages); 
    this._updateLocation(visible.first);
    this.eventBus.dispatch("updateviewarea", {
      source: this,
      location: this._location,
    });
  }
  containsElement(element) {
    return this.container.contains(element);
  }
  focus() {
    this.container.focus();
  }
  get _isScrollModeHorizontal() {
    
    
    return this.isInPresentationMode
      ? false
      : this._scrollMode === ScrollMode.HORIZONTAL;
  }
  get _isContainerRtl() {