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 viewer = appConfig.viewerContainer;
    this.pdfViewer = new PDFViewer({
      container,
      viewer,
      eventBus,
      renderingQueue: pdfRenderingQueue,
      linkService: pdfLinkService,
      downloadManager,
      findController,
      renderer: AppOptions.get("renderer"),
      enableWebGL: AppOptions.get("enableWebGL"),
      l10n: this.l10n,
      textLayerMode: AppOptions.get("textLayerMode"),
      imageResourcesPath: AppOptions.get("imageResourcesPath"),
      renderInteractiveForms: AppOptions.get("renderInteractiveForms"),
      enablePrintAutoRotate: AppOptions.get("enablePrintAutoRotate"),
      useOnlyCssZoom: AppOptions.get("useOnlyCssZoom"),
      maxCanvasPixels: AppOptions.get("maxCanvasPixels"),
      enableScripting: AppOptions.get("enableScripting"),
      mouseState: this._mouseState,
    });
    pdfRenderingQueue.setViewer(this.pdfViewer);
    pdfLinkService.setViewer(this.pdfViewer);
    this.pdfThumbnailViewer = new PDFThumbnailViewer({
      container: appConfig.sidebar.thumbnailView,
      eventBus,
      renderingQueue: pdfRenderingQueue,
      linkService: pdfLinkService,
      l10n: this.l10n,
    });