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
this._pages = [];
    this._currentPageNumber = 1;
    this._currentScale = UNKNOWN_SCALE;
    this._currentScaleValue = null;
    this._pageLabels = null;
    this._buffer = new PDFPageViewBuffer(DEFAULT_CACHE_SIZE);
    this._location = null;
    this._pagesRotation = 0;
    this._optionalContentConfigPromise = null;
    this._pagesRequests = new WeakMap();
    this._firstPageCapability = createPromiseCapability();
    this._onePageRenderedCapability = createPromiseCapability();
    this._pagesCapability = createPromiseCapability();
    this._scrollMode = ScrollMode.VERTICAL;
    this._spreadMode = SpreadMode.NONE;
    if (this._onBeforeDraw) {
      this.eventBus._off("pagerender", this._onBeforeDraw);
      this._onBeforeDraw = null;
    }
    if (this._onAfterDraw) {
      this.eventBus._off("pagerendered", this._onAfterDraw);
      this._onAfterDraw = null;
    }
    this._resetScriptingEvents();
    
    this.viewer.textContent = "";
    
    this._updateScrollMode();
  }
  _scrollUpdate() {