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
eventBus._on("nextpage", webViewerNextPage);
    eventBus._on("previouspage", webViewerPreviousPage);
    eventBus._on("zoomin", webViewerZoomIn);
    eventBus._on("zoomout", webViewerZoomOut);
    eventBus._on("zoomreset", webViewerZoomReset);
    eventBus._on("pagenumberchanged", webViewerPageNumberChanged);
    eventBus._on("scalechanged", webViewerScaleChanged);
    eventBus._on("rotatecw", webViewerRotateCw);
    eventBus._on("rotateccw", webViewerRotateCcw);
    eventBus._on("optionalcontentconfig", webViewerOptionalContentConfig);
    eventBus._on("switchscrollmode", webViewerSwitchScrollMode);
    eventBus._on("scrollmodechanged", webViewerScrollModeChanged);
    eventBus._on("switchspreadmode", webViewerSwitchSpreadMode);
    eventBus._on("spreadmodechanged", webViewerSpreadModeChanged);
    eventBus._on("documentproperties", webViewerDocumentProperties);
    eventBus._on("find", webViewerFind);
    eventBus._on("findfromurlhash", webViewerFindFromUrlHash);
    eventBus._on("updatefindmatchescount", webViewerUpdateFindMatchesCount);
    eventBus._on("updatefindcontrolstate", webViewerUpdateFindControlState);
    if (AppOptions.get("pdfBug")) {
      _boundEvents.reportPageStatsPDFBug = reportPageStatsPDFBug;
      eventBus._on("pagerendered", _boundEvents.reportPageStatsPDFBug);
      eventBus._on("pagechanging", _boundEvents.reportPageStatsPDFBug);
    }
    if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
      eventBus._on("fileinputchange", webViewerFileInputChange);
      eventBus._on("openfile", webViewerOpenFile);
    }
  },
  bindWindowEvents() {