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
}
    }
    if (pdfTitle) {
      this.setTitle(
        `${pdfTitle} - ${contentDispositionFilename || document.title}`
      );
    } else if (contentDispositionFilename) {
      this.setTitle(contentDispositionFilename);
    }
    if (info.IsXFAPresent && !info.IsAcroFormPresent) {
      console.warn("Warning: XFA is not supported");
      this._delayedFallback(UNSUPPORTED_FEATURES.forms);
    } else if (
      (info.IsAcroFormPresent || info.IsXFAPresent) &&
      !this.pdfViewer.renderInteractiveForms
    ) {
      console.warn("Warning: Interactive form support is not enabled");
      this._delayedFallback(UNSUPPORTED_FEATURES.forms);
    }
    
    let versionId = "other";
    if (KNOWN_VERSIONS.includes(info.PDFFormatVersion)) {
      versionId = `v${info.PDFFormatVersion.replace(".", "_")}`;
    }
    let generatorId = "other";
    if (info.Producer) {
      const producer = info.Producer.toLowerCase();
      KNOWN_GENERATORS.some(function (generator) {
        if (!producer.includes(generator)) {
          return false;