pert:
The popup steals screen focus even if Arduino IDE is running in the background, there is no way to ignore an update, and the UI is confusing.
It indeed takes some real estate away from the screen and is 'always on top'. For me the worse part is that the IDE becomes very unresponsive from about 10 seconds before the popup appears. Happily hacking code and nothing happens.
pert:
I guess that would be File > Preferences > Check for updates on startup? I had thought that preference only referred to updates to the IDE, good to know, thanks!
Yes, that is the one. It was actually pure coincidence that I tried it just before the question came up.