Listening to some music with the default visualiser turned on as usual... and the cat decided to walk over the keyboard
The display behaviour changed markedly, so I went googling. A bit of a circular trip eventuated, in particular via here, pointing me to hitting the "?" with the visualiser running.
It's brilliant... I love having the nebulosity at full whack ("N" key), and there's an impressive range of modes available ("M" key).
All it needs now is a way to feed the visualiser to a decent-sized display via AppleTV...