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
isValidScrollMode,
  isValidSpreadMode,
  MAX_AUTO_SCALE,
  moveToEndOfArray,
  NullL10n,
  PresentationModeState,
  RendererType,
  SCROLLBAR_PADDING,
  scrollIntoView,
  ScrollMode,
  SpreadMode,
  TextLayerMode,
  UNKNOWN_SCALE,
  VERTICAL_PADDING,
  watchScroll,
} from "./ui_utils.js";
import { PDFRenderingQueue, RenderingStates } from "./pdf_rendering_queue.js";
import { AnnotationLayerBuilder } from "./annotation_layer_builder.js";
import { PDFPageView } from "./pdf_page_view.js";
import { SimpleLinkService } from "./pdf_link_service.js";
import { TextLayerBuilder } from "./text_layer_builder.js";
const DEFAULT_CACHE_SIZE = 10;
/**
 * @typedef {Object} PDFViewerOptions
 * @property {HTMLDivElement} container - The container for the viewer element.
 * @property {HTMLDivElement} [viewer] - The viewer element.
 * @property {EventBus} eventBus - The application event bus.
 * @property {IPDFLinkService} linkService - The navigation/linking service.
 * @property {DownloadManager} [downloadManager] - The download manager
 *   component.