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
[10, 20],
            [10, 20],
            [10, 20],
            [10, 20],
          ],
          [tallPage, shortPage, tallPage, tallPage],
          [
            [10, 50],
            [10, 50],
            [10, 50],
            [10, 50],
          ],
          [
            [10, 20],
            [10, 20],
            [10, 20],
            [10, 20],
          ],
        ]);
        
        const bsResult = 6;
        expect(
          backtrackBeforeAllVisibleElements(bsResult, pages, top1)
        ).toEqual(4);
      });
      it("handles case 3", function () {
        const pages = makePages([
          [
            [10, 20],
            [10, 20],