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
ProgressBar,
  RendererType,
  ScrollMode,
  SidebarView,
  SpreadMode,
  TextLayerMode,
} from "./ui_utils.js";
import { AppOptions, OptionKind } from "./app_options.js";
import {
  build,
  createPromiseCapability,
  getDocument,
  getFilenameFromUrl,
  GlobalWorkerOptions,
  InvalidPDFException,
  LinkTarget,
  loadScript,
  MissingPDFException,
  OPS,
  PDFWorker,
  PermissionFlag,
  shadow,
  UnexpectedResponseException,
  UNSUPPORTED_FEATURES,
  version,
} from "pdfjs-lib";
import { CursorTool, PDFCursorTools } from "./pdf_cursor_tools.js";
import { PDFRenderingQueue, RenderingStates } from "./pdf_rendering_queue.js";
import { OverlayManager } from "./overlay_manager.js";
import { PasswordPrompt } from "./password_prompt.js";