This is regarding  issue 840176  where Chrome was inadvertently triggering the login dialog interstitial for an HTTP authentication challenge that was received for a subresource of the main frame.

There are already browser tests for verifying that the interstitial is invoked for cross origin main frame requests (e.g. here). But there were no tests to verify that the interstitial DOESN'T show up for a cross origin sub-resource request. Such a test would've caught this regression sooner.

Because of  issue 840176  we now know that at least one web framework and therefore the apps that are built using it relies on this behavior. We should try to not regress it by adding a browser test.