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 button = document.createElement("button");
      const buttonClicked = waitOnEventOrTimeout({
        target: button,
        name: "click",
        delay: 10,
      });
      
      buttonClicked.then(function (type) {
        expect(type).toEqual(WaitOnType.TIMEOUT);
        done();
      }, done.fail);
    });
    it("should resolve on event, using the EventBus", function (done) {
      const pageRendered = waitOnEventOrTimeout({
        target: eventBus,
        name: "pagerendered",
        delay: 10000,
      });
      
      eventBus.dispatch("pagerendered");
      pageRendered.then(function (type) {
        expect(type).toEqual(WaitOnType.EVENT);
        done();
      }, done.fail);
    });
    it("should resolve on timeout, using the EventBus", function (done) {
      const pageRendered = waitOnEventOrTimeout({
        target: eventBus,
        name: "pagerendered",
        delay: 10,