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
import { PDFAttachmentViewer } from "./pdf_attachment_viewer.js";
import { PDFDocumentProperties } from "./pdf_document_properties.js";
import { PDFFindBar } from "./pdf_find_bar.js";
import { PDFFindController } from "./pdf_find_controller.js";
import { PDFHistory } from "./pdf_history.js";
import { PDFLayerViewer } from "./pdf_layer_viewer.js";
import { PDFLinkService } from "./pdf_link_service.js";
import { PDFOutlineViewer } from "./pdf_outline_viewer.js";
import { PDFPresentationMode } from "./pdf_presentation_mode.js";
import { PDFSidebar } from "./pdf_sidebar.js";
import { PDFSidebarResizer } from "./pdf_sidebar_resizer.js";
import { PDFThumbnailViewer } from "./pdf_thumbnail_viewer.js";
import { PDFViewer } from "./pdf_viewer.js";
import { SecondaryToolbar } from "./secondary_toolbar.js";
import { Toolbar } from "./toolbar.js";
import { viewerCompatibilityParams } from "./viewer_compatibility.js";
import { ViewHistory } from "./view_history.js";
const DEFAULT_SCALE_DELTA = 1.1;
const DISABLE_AUTO_FETCH_LOADING_BAR_TIMEOUT = 5000; 
const FORCE_PAGES_LOADED_TIMEOUT = 10000; 
const WHEEL_ZOOM_DISABLED_TIMEOUT = 1000; 
const ENABLE_PERMISSIONS_CLASS = "enablePermissions";
const ViewOnLoad = {
  UNKNOWN: -1,
  PREVIOUS: 0, 
  INITIAL: 1,
};
const ViewerCssTheme = {
  AUTOMATIC: 0, 
  LIGHT: 1,