Interesting. How "similar"* are the two TVs? *Age, manufacturer, etc. Besides resolution (4k), the screens are from different manufacturers and are 3-5 year manufacturing dates apart. Huh. Further clarifying, what "state" is the machine tending to "live" in? In other words, which screen is it claiming to be attached to "most" of the time (while the display is off)? The physical screen is the one that is 'final' after the notification cycle. Is the machine supposed to be asleep? I suppose it should be! The only thing to note is that the 'turn display off after inactivity' setting is disabled, along with disabling screensaver.

Thanks for your continued exploration. The screens are both HDMI TV's, no USB hub. Both have different sets of USB / Bluetooth devices attached. The gap between these events is every 2-10 seconds. CGDisplayIsActive and Online returns true and Asleep returns false for all screen representations, physical or virtual. Bug report number is FB17969822.

Sorry to continue, but I'm just stepping back slightly. Thanks to your explanation I'm now able to identify this virtual screen and ignore it, however this doesn't solve the underlying issue that I'm getting hundreds of these notifications while the screen is turned off. These notifications cycle between replacing the primary screen with this virtual screen and back again, so half of the notifications I'm still needing to process further to check if the physical screens have changed. This seems somewhat inefficient. This behavior is reproducible via two Mac minis of different generations, with different screens. I'm unsure if this is a macOS display detection bug or not, so I've submitted it via Feedback Assistant. Any further context here would be greatly appreciated!

Wonderful answer - thank you very much for the detailed response! Very interesting :-) Did I miss something in the documentation around this particular behavior with the virtual screen notification churn? If not, perhaps I'll open a feedback request to improve the documentation. I'm sure a few apps utilise this particular notification, and could unknowingly be doing something inefficiently. I only discovered this behavior in my own app after having recently purchased a Mac mini. My laptop does not exhibit this behaviour for example, so this wouldn't immediately be obvious to a developer. Thanks again!