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._onBeforeDraw = evt => {
      const pageView = this._pages[evt.pageNumber - 1];
      if (!pageView) {
        return;
      }
      
      
      this._buffer.push(pageView);
    };
    this.eventBus._on("pagerender", this._onBeforeDraw);
    this._onAfterDraw = evt => {
      if (evt.cssTransform || this._onePageRenderedCapability.settled) {
        return;
      }
      this._onePageRenderedCapability.resolve();
      this.eventBus._off("pagerendered", this._onAfterDraw);
      this._onAfterDraw = null;
    };
    this.eventBus._on("pagerendered", this._onAfterDraw);
    
    
    firstPagePromise
      .then(firstPdfPage => {
        this._firstPageCapability.resolve(firstPdfPage);
        this._optionalContentConfigPromise = optionalContentConfigPromise;
        const scale = this.currentScale;
        const viewport = firstPdfPage.getViewport({ scale: scale * CSS_UNITS });
        const textLayerFactory =
          this.textLayerMode !== TextLayerMode.DISABLE ? this : null;