Two-page scrolling

[SieC65]

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

[SieC65]

Please let me know if this is a known mistake, or if it is not considered a mistake at all.
Should I hope that it will be fixed in the future?

Just to be sure that I clearly explain the problem, I attach 2 screenshots. I expect to see pages 4-7 after the first press of a key "Page Down". But I have to press it 4 times, of which the first three times nothing happens.

[Tsudico]

I have also noticed this behavior. When showing multiple pages, you must remember which "page" you are on when using previous/next page keys because they operate on a page level and not on the view level.

This seems to be an issue due to the following lines in web/app.js:

function webViewerNextPage() {
  PDFViewerApplication.page++;
}
function webViewerPreviousPage() {
  PDFViewerApplication.page--;
}

I think it could possibly be fixed by adjusting to the following code:

function webViewerNextPage() {
  PDFViewerApplication.page += PDFViewerApplication.pdfViewer._getVisiblePages().views.length;
}
function webViewerPreviousPage() {
  PDFViewerApplication.page -= PDFViewerApplication.pdfViewer._getVisiblePages().views.length;
}

But there probably is a better way.

[ikartikgautam]

Hii, I will be creating a pull request soon, to fix this issue. Currently I am thoroughly testing it.

[Snuffleupagus]

Please note that the changes suggested in #11946 (comment), for a variety of reasons, are unfortunately not a good idea in general and that a proper solution will be more involved than it may look at first (which is why this issue is still open).

[ikartikgautam]

okay thanks, i was about to create a pull request.