Make print() run synchronously

[TimothyGu]

This was a slip-up in the larger clean-up commit in 61da493, where "synchronous" was changed to "in parallel". All browsers implement the synchronous behavior.

Fixes #4686.

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/timers-and-user-prompts.html ( diff )

[TimothyGu]

I’m not sure quite to test this by the way, nor does there seem to be any existing tests for print() in WPT.

[domenic]

I think you could test at least beforeprint by calling preventDefault. Testing afterprint would require a manual test so that someone could cancel out of the print dialog. /cc @gsnedders in case he knows things about testing print dialogs.

[domenic]

We should also, from https://html.spec.whatwg.org/multipage/parsing.html#the-end:print-when-loaded, queue a task to run the printing steps, instead of running them directly. Right?

[TimothyGu]

I think you could test at least beforeprint by calling preventDefault.

This doesn't seem to work, as printing steps don't respect preventDefault either in spec or in implementations.

We should also, from https://html.spec.whatwg.org/multipage/parsing.html#the-end:print-when-loaded, queue a task to run the printing steps, instead of running them directly. Right?

I think you are right from some light testing, though that seems to be an existing bug. In particular, the following

<script>
  onpageshow = () => { console.log('pageshow', document.readyState); };
  onload = () => { console.log('load', document.readyState); };
  onbeforeprint = (e) => { console.log('beforeprint', document.readyState); e.preventDefault(); };
  print();
  console.log('after print() returns');
</script>

prints

after print() returns
load complete
pageshow complete
beforeprint complete

in all the browsers I can test.

[domenic]

Sorry, I forgot the purpose of beforeprint; you're right, it's not cancelable.

So yeah, I'm happy to land this when we fix the preexisting bug, plus file an issue for adding manual tests and/or figuring out how to automate them.

[gsnedders]

Pretty sure there's no way to test that it happens synchronously.

[zcorpan]

Please make sure there's an issue in wpt with label type:untestable when you run into things that can't be automated, and document why you need it.

This issue seems related at least web-platform-tests/wpt#5667

[TimothyGu]

Test added in web-platform-tests/wpt#17439.

[domenic]

LGTM, with tests in progress. Let's merge this.