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
PDFViewerApplication.pdfViewer.scrollMode = evt.mode;
}
function webViewerSwitchSpreadMode(evt) {
  PDFViewerApplication.pdfViewer.spreadMode = evt.mode;
}
function webViewerDocumentProperties() {
  PDFViewerApplication.pdfDocumentProperties.open();
}
function webViewerFind(evt) {
  PDFViewerApplication.findController.executeCommand("find" + evt.type, {
    query: evt.query,
    phraseSearch: evt.phraseSearch,
    caseSensitive: evt.caseSensitive,
    entireWord: evt.entireWord,
    highlightAll: evt.highlightAll,
    findPrevious: evt.findPrevious,
  });
}
function webViewerFindFromUrlHash(evt) {
  PDFViewerApplication.findController.executeCommand("find", {
    query: evt.query,
    phraseSearch: evt.phraseSearch,
    caseSensitive: false,
    entireWord: false,
    highlightAll: true,
    findPrevious: false,
  });
}
function webViewerUpdateFindMatchesCount({ matchesCount }) {