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._goToPreviousPage();
        } else if (delta < 0) {
          this._goToNextPage();
        }
        break;
    }
  }
  /**
   * @private
   */
  _addWindowListeners() {
    this.showControlsBind = this._showControls.bind(this);
    this.mouseDownBind = this._mouseDown.bind(this);
    this.mouseWheelBind = this._mouseWheel.bind(this);
    this.resetMouseScrollStateBind = this._resetMouseScrollState.bind(this);
    this.contextMenuBind = this._contextMenu.bind(this);
    this.touchSwipeBind = this._touchSwipe.bind(this);
    window.addEventListener("mousemove", this.showControlsBind);
    window.addEventListener("mousedown", this.mouseDownBind);
    window.addEventListener("wheel", this.mouseWheelBind, { passive: false });
    window.addEventListener("keydown", this.resetMouseScrollStateBind);
    window.addEventListener("contextmenu", this.contextMenuBind);
    window.addEventListener("touchstart", this.touchSwipeBind);
    window.addEventListener("touchmove", this.touchSwipeBind);
    window.addEventListener("touchend", this.touchSwipeBind);
  }
  /**
   * @private
   */
  _removeWindowListeners() {