It's a UNIX (or at least UNIX-like) core OS. There's no reason User 1's jobs can't continue while User 2 is controlling the UI. As to resource contention, it can simply come down to first-come-first-served.
Now I'll agree that I'm uncertain as to whether it's a good idea -- the fact that I can cite a specific example of why it can be seen as a "problem" doesn't mean that "fixing" it wouldn't introduce a bigger issue -- but the technical aspects behind it aren't a real problem, given the multitasking nature of the underlying OS core.
Well, all software is a set of choices and also you’ll find security as a consideration in that, especially when it comes to different user spaces. And yes, I’m sure that software can be made to do just about anything you can conceivably imagine.
But, not everything that can be conceivably imagined and not everything that can be conceivably accomplished is going to be done. That’s a judgment call on behalf of the people who make the software.
It’s also like I said earlier about software improvements and users desires..., in that if enough users really want something, Apple will incorporate that into the software, unless there is a problem (i.e., requiring a lot more resources and work to get it accomplished and they’ve got a lot of other things “on the table” to get done first).
If there is not widespread demand for such things — I would not hold out any hope for it to get done, even if it is technically feasible. That’s because it can be confusing and/or disconcerting to people to find that something from one user space is continuing to operate while they are on another user space. You have to also consider that Apple is about the “User Interface” and I’m not sure if that is something good for the User Interface, anyway...
You and I may understand technically what is going on. Perhaps a majority of users would be totally confused as to what is going on and not understand that even though they are in one User space, that an operation is going on in another User space and they don’t know how to stop it. I can see that generating many complaints, actually... LOL...
I should add something here...
You said — It’s a UNIX (or at least UNIX-like) core OS. There’s no reason User 1’s jobs can’t continue while User 2 is controlling the UI. As to resource contention, it can simply come down to first-come-first-served.
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In case you don’t know and/or others don’t know this — other jobs *do continue* in the other User space. It’s just that iTunes doesn’t keep playing music while one is in a different User space than the one in which iTunes is playing.
I think it comes down to a couple of things, perhaps security and perhaps the User Interface guidelines that Apple has.