Add a pref to postMessage when mozPrintCallback has completed (bug 2036265)

[calixteman]

Summary

• Adds a new postMessageAfterPrintCallback browser option (default false).
• When enabled by Firefox via the pdfjs.postMessageAfterPrintCallback pref, web/firefox_print_service.js posts "ready" or "error" to the window after each page's mozPrintCallback resolves.
• Allows embedders (e.g. print-preview test harnesses) to observe when print rendering for a page has actually finished, instead of relying on STATE_STOP.

This upstreams the viewer-side portion of Phabricator D297837 (original patch by @dholbert). The Firefox-side pref wiring (PdfJsDefaultPrefs.js, PdfStreamConverter.sys.mjs) lives in mozilla-central; only the PDF.js bits land here so they aren't overwritten at the next pdf.js import.

See bug 2036265 for context.

Test plan

• npx gulp lint passes.
• With pdfjs.postMessageAfterPrintCallback flipped on in Firefox, printing a PDF results in one window.postMessage("ready", "*") per page on success and "error" on failure.
• Default behavior (pref off) is unchanged.

🤖 Generated with Claude Code

[calixteman]

@dholbert, is there anything you want to add ?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.16%. Comparing base (5a4d93a) to head (567f585).
⚠️ Report is 48 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21308      +/-   ##
==========================================
- Coverage   81.33%   80.16%   -1.18%     
==========================================
  Files         256      257       +1     
  Lines       65138    66843    +1705     
==========================================
+ Hits        52979    53582     +603     
- Misses      12159    13261    +1102     

Flag	Coverage Δ
integrationtest	67.00% <ø> (+0.10%)	⬆️
unittest	56.87% <ø> (+0.02%)	⬆️
unittestcli	56.07% <ø> (+<0.01%)	⬆️

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[dholbert]

Thank you @calixteman! Sorry, I'd been meaning to turn this into a PR - I appreciate you doing it for me.

@dholbert, is there anything you want to add ?

No, I think I'm good with this as-is without adding other things (aside from addressing review comments).

(I had started exploring formalizing this as a per-page postMessage callback, and making sure that worked properly as a way to test print-preview of multi-page PDFs - that's what I was sort of trying to sort out before posting a PR. But I didn't yet get that working end-to-end with the tests that I was writing for print-preview of multi-page PDFs -- and in any case that's not something I was as concerned with since the Firefox print-preview tests are largely single-page anyway, for better or for worse. We can always add those tests later on, in any case.)

@Snuffleupagus's review feedback looks valid (likely just accounting for me not knowing what I'm doing in this code). I can look at addressing that and post a diff (not sure if I can push updates to the PR directly since it's owned by @calixteman)

[dholbert]

@calixteman here's a patch (which should apply cleanly on top of this PR) that I think addresses @Snuffleupagus's review feedback - would you mind updating the PR with this?

0001-address-review-feedback.patch

I tested the same changes in my firefox clone (with the PdfStreamConverter.sys.mjs change reverted as well, per @Snuffleupagus's suggestion) and I confirmed that my test still runs as-expected -- i.e. the postMessage is getting sent as I expect it to, if-and-only-if this testing pref is set.

[calixteman]

@calixteman here's a patch (which should apply cleanly on top of this PR) that I think addresses @Snuffleupagus's review feedback - would you mind updating the PR with this?

0001-address-review-feedback.patch

I don't really know if we can commandeer a PR as we do on Phab... so I added you as a co-author.

[Snuffleupagus]

Should this also be done when printing XFA documents?
If so, the following branch needs to be updated as well:

pdf.js/web/firefox_print_service.js

Lines 169 to 172 in e766198

	if (pdfDocument.isPureXfa) {
	getXfaHtmlForPrinting(printContainer, pdfDocument);
	return;
	}

[calixteman]

Should this also be done when printing XFA documents? If so, the following branch needs to be updated as well:

pdf.js/web/firefox_print_service.js

Lines 169 to 172 in e766198

	if (pdfDocument.isPureXfa) {
	getXfaHtmlForPrinting(printContainer, pdfDocument);
	return;
	}

I don't know if we want to text printing XFA so it's probably useless but it probably doesn't hurt to have the "feature" just in case it's useful.

[dholbert]

Yeah, until/unless we have print-preview tests for XFA in Firefox (we don't currently), we could go either way on that branch; doesn't matter much to me.

[dholbert]

@calixteman / @Snuffleupagus is this good to be merged? Let me know if there's anything else I can do to help here. (I'm looking forward to being able to land a patch that adds some tests in https://bugzilla.mozilla.org/show_bug.cgi?id=2022056 , once this PR is merged and then synced into Firefox.)