I also teach from an iPad, as do the majority of the teachers at my school. I don't think the issue is as simple as the article presents.
> Essentially, you had to swipe up as though going to the home screen, then tap on the app you wanted in the dock, and that app would take over the external display if it had a presentation mode. This was weird and hard to understand, but Apple’s solution is to just make it so that whatever is the most recently tapped app takes over, which is much, much worse.
Some apps, like PowerPoint and Google Slides, only enter "presenter mode" and take over the external AirPlay screen on explicit user action.
Other apps, like Notability, will automatically enter "presenter mode" and take over the external display if they detect a secondary display without explicit user action, even when it is not the only app on screen. You can then turn off this mode if you would like by going into the menus.
AirPlay calls itself "screen mirroring" in the Apple menus, and I can say with confidence from talking to the average teacher in the building that they expect it to mirror their screen. Well-behaved apps like PowerPoint that only take over the AirPlay session when prompted by the user will match the naïve user's expectations.
Apps like Notability that take over the entire AirPlay session without explicit interaction and without being the only app on screen break the naïve user's expectations of display mirroring. Multiple teachers have asked me why their calculator app (for example) does not show up on the Airplay display when they're using Notability.
> As for what the behavior should be, I’m not a developer, but here’s a proposal: if you’re using an app that is in presenter mode (e.g., PowerPoint playing a presentation, or a GoodNotes document open), it should always get precedence on the external display, until that presentation gets stopped, the document is closed, or the app is quit. Period.
I understand and even share the author's concerns, but I think the new compromise—only the active app can take over external displays—makes sense as a default because it matches the average user's expectations of AirPlay. The old method had major benefits and some annoyances, and in my opinion it should still be an option for a background app to control the external display if the user explicitly wants it or has enabled some toggle.
Apple did not handle the 26-27 transition well at all, and this really threw me the first time it came up. (I did not have the option to not update my employer's iPad, so I've been on 27 since release.) There should be a way to have a second app (secretly) open and focused while you present only from app #1, but I think the new default is better.