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
fileInput.setAttribute("type", "file");
    fileInput.oncontextmenu = noContextMenuHandler;
    document.body.appendChild(fileInput);
    if (
      !window.File ||
      !window.FileReader ||
      !window.FileList ||
      !window.Blob
    ) {
      appConfig.toolbar.openFile.setAttribute("hidden", "true");
      appConfig.secondaryToolbar.openFileButton.setAttribute("hidden", "true");
    } else {
      fileInput.value = null;
    }
    fileInput.addEventListener("change", function (evt) {
      const files = evt.target.files;
      if (!files || files.length === 0) {
        return;
      }
      PDFViewerApplication.eventBus.dispatch("fileinputchange", {
        source: this,
        fileInput: evt.target,
      });
    });
    
    appConfig.mainContainer.addEventListener("dragover", function (evt) {
      evt.preventDefault();
      evt.dataTransfer.dropEffect = "move";
    });
    appConfig.mainContainer.addEventListener("drop", function (evt) {