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"The current reason is not so much to do with the UI"

It has a great deal to do with UI.

I disagree. Layering a new UI on top of an existing OS is a lot of work, but ultimately thats all it is - another layer. What iOS and Android do that is fundamentally different to OSX and other desktop OSs is not so much to do with the UI, its to do with quickly killing or hibernating any process that isn't being used right at that time.

When you learn to write an app for iOS or Android, the first thing you learn is that your app needs to be ready to freeze and save state at a moments notice, and then re-hydrate itself later, also at a moments notice. This is the fundamental difference in the OS, not the touchscreen and widgets.

This app-nap functionality is available in OS X too so long as the app implements the correct APIs, which many do. This was introduced in 10.8 as part of their performance effort I believe.

This is seen in OS X when an app window appears greyed out with a progress spinner in the center. The app may start out in this state if the machine was hibernated, or it may go into this state if it is open but it fits certain criteria, like all its windows being occluded by others. When this happens, the process is killed.

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