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
await new Promise(resolve => {
        this.eventBus._on(
          "documentloaded",
          evt => {
            resolve();
          },
          { once: true }
        );
      });
      if (pdfDocument !== this.pdfDocument) {
        return; 
      }
    }
    const updateFromSandbox = ({ detail }) => {
      const { id, command, value } = detail;
      if (!id) {
        switch (command) {
          case "clear":
            console.clear();
            break;
          case "error":
            console.error(value);
            break;
          case "layout":
            this.pdfViewer.spreadMode = apiPageLayoutToSpreadMode(value);
            break;
          case "page-num":