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
const typedArray = new Uint8Array([1, 2, 3, 4, 5]);
      const dataUrl = createObjectURL(
        typedArray,
        "application/pdf",
         true
      );
      
      expect(dataUrl.startsWith("data:")).toEqual(true);
      expect(getPDFFileNameFromURL(dataUrl + "?file1.pdf")).toEqual(
        "document.pdf"
      );
      
      expect(getPDFFileNameFromURL("     " + dataUrl + "?file2.pdf")).toEqual(
        "document.pdf"
      );
    });
  });
  describe("EventBus", function () {
    it("dispatch event", function () {
      const eventBus = new EventBus();
      let count = 0;
      eventBus.on("test", function (evt) {
        expect(evt).toEqual(undefined);
        count++;
      });
      eventBus.dispatch("test");
      expect(count).toEqual(1);
    });
    it("dispatch event with arguments", function () {
      const eventBus = new EventBus();