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 LOADINGBAR_END_OFFSET_VAR = "--loadingBar-end-offset";
const PresentationModeState = {
  UNKNOWN: 0,
  NORMAL: 1,
  CHANGING: 2,
  FULLSCREEN: 3,
};
const SidebarView = {
  UNKNOWN: -1,
  NONE: 0,
  THUMBS: 1, 
  OUTLINE: 2,
  ATTACHMENTS: 3,
  LAYERS: 4,
};
const RendererType = {
  CANVAS: "canvas",
  SVG: "svg",
};
const TextLayerMode = {
  DISABLE: 0,
  ENABLE: 1,
  ENABLE_ENHANCE: 2,
};
const ScrollMode = {
  UNKNOWN: -1,
  VERTICAL: 0, 
  HORIZONTAL: 1,
  WRAPPED: 2,
};