I'm not sure what I did, but when I click on an alert in the dashboard, it takes me to the rule/monitor definition and not the details of the alert I'm clicking on. the URL looks right, it says "drilldown" and has the GUID of the alert ID.
It looks like somehow I nuked the Alert perspective and there’s no way I can see to get it back. I restored from backup which restored it and copied out the JSON of that perspective in case it happens again.
It would be nice if the undocumented modules had some way to revert back to defaults without having to restore from backup.