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
if (!viewer) {
      return;
    }
    const container = viewer.parentNode;
    const scrollbarWidth = container.offsetWidth - viewer.offsetWidth;
    if (scrollbarWidth > 0) {
      const doc = document.documentElement;
      doc.style.setProperty(LOADINGBAR_END_OFFSET_VAR, `${scrollbarWidth}px`);
    }
  }
  hide() {
    if (!this.visible) {
      return;
    }
    this.visible = false;
    this.bar.classList.add("hidden");
  }
  show() {
    if (this.visible) {
      return;
    }
    this.visible = true;
    this.bar.classList.remove("hidden");
  }
}
/**
 * Moves all elements of an array that satisfy condition to the end of the
 * array, preserving the order of the rest.
 */
function moveToEndOfArray(arr, condition) {