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
expect(count).toEqual(1);
        document.removeEventListener("test", domEventListener);
        done();
      });
    });
  });
  describe("isValidRotation", function () {
    it("should reject non-integer angles", function () {
      expect(isValidRotation()).toEqual(false);
      expect(isValidRotation(null)).toEqual(false);
      expect(isValidRotation(NaN)).toEqual(false);
      expect(isValidRotation([90])).toEqual(false);
      expect(isValidRotation("90")).toEqual(false);
      expect(isValidRotation(90.5)).toEqual(false);
    });
    it("should reject non-multiple of 90 degree angles", function () {
      expect(isValidRotation(45)).toEqual(false);
      expect(isValidRotation(-123)).toEqual(false);
    });
    it("should accept valid angles", function () {
      expect(isValidRotation(0)).toEqual(true);
      expect(isValidRotation(90)).toEqual(true);
      expect(isValidRotation(-270)).toEqual(true);
      expect(isValidRotation(540)).toEqual(true);
    });
  });
  describe("isPortraitOrientation", function () {
    it("should be portrait orientation", function () {
      expect(
        isPortraitOrientation({