SQL 2016 SP1 Mirroring - slow database list refresh and repetitive automatic failovers
It looks like CU1 corrects the issue I was having with the spontaneous failovers.  The mirror now seems stable and can fail manually back and forth correctly.

Refreshing the database list on the secondary is still slow, presumably because it is trying to log into the database but can't; log files contain "Login failed for user. Reason: Failed to open the explicitly specified database 'MirrorTest' when I try to refresh the list.