This is better, but I do not think this is enough.

not dev stage: console.error with full debug info, along with console.warn containing clue how to customize it

The problem that prompted the issue is actual users complaining. The users are on mobile phones.
In the above case, the situation would not improve. Mobile phone users do (can?) not look at the console to figure out what's going on, and are getting confused by the lack of feedback.

Thus, my suggestion is to reload the same page by default in production mode. This is when actual feedback can be given to actual users, instead of no feedback at all, which is the worst possible behavior IMHO

It's the developer's responsibility to implement faces.ajax.addOnError conform business requirements, not the framework's.

Note that ExceptionHandler has same issue. There's nothing in prod stage. If we change this then ExceptionHandler also needs change. And possibly many other things which only appear in dev stage. I'd rather not start this here. Also note that this dev-stage-only alert is part of the spec: https://jakarta.ee/specifications/faces/4.0/jakarta-faces-4.0#a6806

If the project stage is “development” (see Determining An Application’s Project Stage) use JavaScript “alert” to signal the error(s).

It's the developer's responsibility to implement faces.ajax.addOnError conform business requirements.

Agree.

Note that ExceptionHandler has same issue.

There is a big difference here. Server-side exceptions can be seen in production logs, debugged, etc.
The reason this took so long to debug for me is that I could not figure out why the users were "hanging"
There are no logs, no error messages, nothing to determine where the errors are coming from in production.
This error is very insidious and it took me weeks to figure out what's going on.

The improvement in current PR should prevent/reduce that in the future.

Yes, I agree it's better, but still the mobile phone case if not handled, since console errors cannot be seen on the phone.

This is especially true when testing is done in the development mode, and production is obviously in "production" mode where the issue rears its ugly head :)

You can decorate console.error to trigger a REST endpoint which captures and logs the message in server side. It'll solve many more mysteries than only a missing faces.ajax.addOnError.

True, but it will slow down the app... especially on mobile. Not sure it's a good solution :)

It'll only slow down if your app spits extremely a lot of console.error lines. Even then you can still send with delay and reset delay when a new line is added. It's not impossible and definitely super helpful. You'll see :)

Agreed, but I think redirect-to-self is the most sensible and simplest option :)

Then implement your faces.ajax.addOnError accordingly.

Of course, but I am trying to argue for the most sensible default here. Not everyone has the time to debug this stuff :)

Ok, at least this can be caught more easily this way

I added a check+trigger on window.onerror so any 3rd party monitoring tool is at least aware.