Reload PDF without losing scroll position?

Question 1: Is it possible to make PDFViewerApplication to reload the PDF without losing its scroll position?

I was able to trigger a reload by using PDFViewerApplication.open(), but the scroll position returns to the top.

If there is no such function, I could save the scrollTop property of #viewerContainer and then set it again after the PDF has been reloaded, but for that I would need to know when the PDF has been rendered.

Question 2: Is there a render-finished callback that I can register?

[allefeld]

Alright, I finally figured it out, and I describe the solution here in case some else needs this.

The default reload behavior of the pdf.js viewer is indeed to preserve the scroll position (and some other GUI states) if the file is exactly the same. However, it doesn’t check the complete file contents (via a hash or fingerprint) but relies on the document ID that is typically (but not necessarily) embedded in a PDF file (see 14.4 in the specification).

In my case, the (contentwise slightly different) PDFs are generated by pdflatex / xelatex / lualatex, and apparently they generate the PDF ID based on the current time and the pathname of the document. Therefore, even if I don’t change anything in the LaTeX document, just process it again to generate a PDF, this PDF has a different ID, and the pdf.js viewer does not treat it as the same document (preserving the scroll position), but as a new one (starting with the scroll position at the top).

Fortunately, there is a way around this. If the environment variable SOURCE_DATE_EPOCH is set to a Unix time stamp (number of seconds since 1 Jan 1970 00:00 UTC), then this time will be used for the PDF ID instead of the current time. Since the pathname of the document doesn’t change either, this ensures an identical ID across regenerations of the PDF, and therefore the viewer shows its scroll-preserving behavior on reload.

[jrkoenig]

I found another solution by overriding the fingerprint inside the PDFDocumentProxy:

// Binary pdf contents stored in data
let doc = await pdfjsLib.getDocument({data}).promise;
// Override the fingerprint after parsing but before passing to the view layer.
// This means that the view state (page, scroll offset, etc.) is preserved.
doc._pdfInfo.fingerprint = 'constantFingerprint';
PDFViewerApplication.load(doc);

Note we have to assign to doc._pdfInfo.fingerprint because doc.fingerprint is a getter. This doesn’t appear to have ill effects despite accessing the presumably private _pdfInfo.

This is similar to the solution in #11496, but doesn’t require patching pdf.worker.js. You could get fancier here and use a fingerprint based on the e.g. basename or path of the file.