Most Proton tests involve launching the server and sending requests to it via preq. Given the fairly minimal public error reporting in node-template-based applications (and even most of that being discarded by the tests), basically all you end up with is a boolean flag of whether the test passed or failed. That's not very helpful, especially for test errors which are locally not reproducible. Either the logs should be preserved as a test artifact, or the server should output more diagnostic details in test mode and the tests should print it on failure.
Yeah, possibly all that this takes is changing [[https://github.com/wikimedia/mediawiki-services-chromium-render/blob/6ba6069eb46e803f9249226dbca0222c34b3cac3/test/utils/server.js#L31|logStream()]] to logStream(true). Not sure whether that would render CI logs unreadable, though. Also I'm not sure where the console ends up, since the server is run via service-runner (so probably a sub-process of some kind?).
As for priority, not sure. I found debugging not locally reproducible issues kind of painful, but it turned out to be sort of PEBKAC in the end (T217113: MediaWiki-Vagrant should use the same Node.js version as Wikimedia production), so maybe these kinds of problems do not occur normally.