There was an error during mirroring failover that seemed to delay the recovery of all the databases on the server.
Login failed for user ‘stash’. Reason: Token-based server access validation failed with an infrastructure error.
So complete failover took about 9 minutes, but 7 of those minutes appear to be due to an issue with one database. Applications were able to connect to databases in the meantime. The only concern is that cross-database queries would not be able to access databases on two different servers.
The solution was to add the login to the mirror server.