Affected versions

• Firefox 84.0a1 (BuildId:20201108212832)

Affected platforms

• Windows 10 64bit
• Ubuntu 20.04 64bit
• macOS 10.14

Preconditions
Enable the following prefs:

• fission.autostart
• gfx.webrender.all

Steps to reproduce

1. Launch Firefox.
2. Access the following link.
3. Open the web console and input window.print().

Expected result

• The print preview is displayed and the content from the Cross origin Iframe is successfully displayed.

Actual result

• The print preview is displayed and the content from the Cross origin Iframe is not displayed. The actual print output seems to be affected as well.

Regression Window

• I'll further investigate asap.

Additional Information

• Please observe the attached screenshot for further information regarding this issue.
• [Suggested Severity] S3
• This issue is reproducible only with Fission enabled.
• The old UI seems to be affected as well (this can be visible on Windows & Linux by opening the print preview via the hamburger menu).
• This issue doesn’t seem to be reproducible on the new ui while opening the print preview via the hamburger menu, CTR+P or via menu bar print option.

Seems like a regression because I thought cross-origin iframes were working in print preview, but now I don't recall testing with window.print(). ni? to Emilio.

Hmm yeah, not sure why window.print() would be different. Ctrl+P works just fine.

So this seems to be a race between the docshell swap that happens when window.print "swaps" into our dialog, and the process swap... If I delay enough the docshell swap like this, then it works:

diff --git a/toolkit/components/printing/content/printUtils.js b/toolkit/components/printing/content/printUtils.js
index cb09179c7146..5e45f88fae30 100644
--- a/toolkit/components/printing/content/printUtils.js
+++ b/toolkit/components/printing/content/printUtils.js
@@ -216,6 +216,8 @@ var PrintUtils = {
       return Promise.reject();
     }
 
+    await new Promise(resolve => setTimeout(() => resolve(), 1000));
+
     // Create a preview browser.
     let args = PromptUtils.objectToPropBag({
       previewBrowser: aExistingPreviewBrowser,

Not sure yet where things are going wrong, though here's a pernosco recording of the problem: https://pernos.co/debug/WWLjbsuKOPlwmRh0pBKX7A/index.html

This doesn't fix the root cause of the issue, but it does fix the issue,
and I think it's worth it, because it's both less work, and less brittle
code (not involving swapDocShells() is always a win in any context).

I'll file a bug for the original problem (which is a Gecko race with the
compositor).

I had to tweak some tests because we insert the preview browser sooner
now, and abortAllDialogs() and such only work after the frame's
DOMContentLoaded event has fired.

Moving my ni? to bug 1676721, where I'll keep digging on the root cause.

As there's no incremental reflow for non-print documents, we can't rely
on FinishReflow getting called.

Not ultra-pretty, better ideas welcome.

https://hg.mozilla.org/mozilla-central/rev/4c8f30ddc6ec

Emilio, what's left to do here?

It looks fixed now, but I still want to land the swapDocShells change, which would be a nice simplification.

Moving to Fission Future as the issue is now fixed and only a nice-to-have simplification is left.

The leave-open keyword is there and there is no activity for 6 months.
:emilio, maybe it's time to close this bug?