It fits Apple's modus operandi to enforce things UI/UX wise, I assume in this case they don't want end-apps to be able to bypass the compositor (and e.g. prevent alerts from showing on the screen or whatnot).
They used to allow it, but they removed the API after 10.6
I guess on modern macOS CGDisplayCapture() is the closest example that still works (although clearly there is still some compositing going on since the orange-dot microphone indicator still appears, and you can get the dock to appear over it if you activate mission control. I'm guessing it does the equivalent of a full-screen window but then tries to lock input somehow).
Apple Employees: Please push back against this behavior. It creates more problems than it solves[0]
The philosophy really creates a lot of problems that end up just frustrating users. It's a really lazy form of "security". Here's a simple example of annoyance: if I have private relay enabled, then turning on my VPN causes an alert (will not dismiss automatically). The same thing happens when I turn it off! It is a fine default, but why can't I decide I want to have automatically dismiss? This is such a silly thing to do considering I can turn off Private Relay, without any privileges, and without any notifications! NOTHING IS BEING SOLVED except annoying users... There's TONS of stupid little bugs that normal people just get used to but have no reason for existing (you can always identify an iPhone user because "i" is never capitalized...)
The philosophy only works if Apple is able to meet everybody's needs, but we're also talking about a company who took years to integrate a flashlight into their smartphone. It's okay that you can't create a perfect system. No one expects you to. It is also okay to set defaults and think about design, that's what people love you for (design). But none of this needs to come at a cost to "power users." I understand we're a small portion of users, but you need to understand that our efforts make your products better. Didn't see an issue with something? That's okay, you're not omniscient. But luckily there's millions of power users who will discover stuff and provide solutions. Even if these solutions are free, you still benefit! Because it makes peoples' experiences better! If it is a really good solution, you can directly integrate it into your product too! Everybody wins! Because designing computers is about designing an ecosystem. You design things people can build on top of! That's the whole fucking point of computers in the first place!
Get your head out of your ass and make awesome products!
[0] If any...
[another example]: Why is it that my Airpods can only connect to one device, constantly switching from my Macbook to iPhone. This often creates unnecessary notifications. I want magic with my headphones. There's no reason I shouldn't be able to be playing Spotify on my computer, walk away from it with my phone in pocket, and have the source switch to my phone. I can pick up my phone and manually change the source. But in reality what happens is I pick up my phone and Spotify pauses because my headphones switched to my iPhone... If you open up access then you bet these things will be solved. I know it means people might be able to then seamlessly switch from their Macbook to their Android, but come on, do you really think you're going to convert them by making their lives harder? Just wait till they try an iPhone! It isn't a good experience... I'm speaking from mine...
For one thing, on most modern systems there isn’t a framebuffer, there’s only a simulation of one created by GPU compositing.
For another, security. Your application doesn’t get to scrape other applications’ windows and doesn’t get to put stuff in them, and they don’t get to do that to yours, unless properly entitled & signed. You can self-sign for development but for distribution that ensures malware can be blocked by having its signature revoked.
All software/hardware in the system (Core Animation, Retina scaling, HDR, Apple Silicon's GPUs) assumes an isolated/composited display model. A model where a modern rendering pipeline is optional would add major complexity, and would also prevent Apple from freely evolving its hardware, system software, and developer APIs.
Additionally, a mandatory compositor blocks several classes of potential security threats, such as malware that could suppress or imitate system UI or silently spy on the user and their applications (at least without user permission).
Why is this the case? What would be the problem with allowing it?