But I don't understand why they're mirroring their desktop to some kind of visual display system? Wouldn't they be using something like a DeckLink for this use-case?

Monitor outputs are for monitors - if you want an application-specific video output you can get that and it won't be a desktop so it won't have this problem.

If you mirror your desktop then yeah... you get whatever desktop UI chrome is on your desktop.

Because most of these displays look like monitors to your system. Using the existing rendering pipelines to a full screen monitor is far easier, and less expensive then custom hardware just to move pixels that the built in graphics card is more than capable of

Ok so it's a quick hack for video output but they're going to run into problems like this. And for example any notifications, system updates, Launchpad, whatever, would also appear. That's how you end up with goofy things like a sign with a Windows 'need to update now' message. If they were doing it properly with a production video output they wouldn't have this problem.

You can call it a "quick hack" if you'd like, but I can assure you that all of the potential pitfalls you mention are taken into account by the people who setup and run these systems.

You're not going to "gotcha!" people who have actually done this stuff.

They're not mirroring their desktop. It's a secondary display running a full screen application.

That's just how this stuff works. Sure, you could use something like a DeckLink, but the vast majority of things like this just use a monitor output.

Thank you! It's crazy to me that anyone that would consider themselves a professional would mirror their desktop for the use cases these people claim to be using. "Mariah Carey playing in front of millions on NYE"? Gimme a break. No one is mirroring the desktop from their Macbook for that performance.

Nobody is mirroring their desktop...

That's how the orange dot is appearing - it's a UI element from the interactive user-interface of macOS, but it's appearing on a video out because they're re-using a monitor signal designed for mirroring or extending you desktop.

That might be pedantic, sure, but the point is that extending is not mirroring.

For the use cases like the one described in the article, that distinction matters. I'm not questioning the usefulness of the orange dot, but rather the people saying the workflow of using a second monitor as output as being invalid and wrong.

You know what I mean and the distinction is meaningless. No one doing a show as big as that is doing it without the hardware necessary to make sure that they can control 100% of the output path.