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
}
        this.touchSwipeState.endX = evt.touches[0].pageX;
        this.touchSwipeState.endY = evt.touches[0].pageY;
        
        
        evt.preventDefault();
        break;
      case "touchend":
        if (this.touchSwipeState === null) {
          return;
        }
        let delta = 0;
        const dx = this.touchSwipeState.endX - this.touchSwipeState.startX;
        const dy = this.touchSwipeState.endY - this.touchSwipeState.startY;
        const absAngle = Math.abs(Math.atan2(dy, dx));
        if (
          Math.abs(dx) > SWIPE_MIN_DISTANCE_THRESHOLD &&
          (absAngle <= SWIPE_ANGLE_THRESHOLD ||
            absAngle >= Math.PI - SWIPE_ANGLE_THRESHOLD)
        ) {
          
          delta = dx;
        } else if (
          Math.abs(dy) > SWIPE_MIN_DISTANCE_THRESHOLD &&
          Math.abs(absAngle - Math.PI / 2) <= SWIPE_ANGLE_THRESHOLD
        ) {
          
          delta = dy;
        }
        if (delta > 0) {