Modern OSes treat the app like a black box, and primarily control its access to resources.

As far as the OS is concerned, it doesn't know or care what pixels the app shows within its rectangle on the screen.

But the OS can mediate access to sensitive resources, like the camera or notifications.

More modern OSes can do things like give you an option to "Grant location access only while using the app."

Imagine if you could systematize this granular functionality of the app down to more granular levels.

Getting a maximum of precise, niche control over subsets of the functionality of the app, where you could grant a bit of location data to one subset of the app but not the other parts of the app.

In the limit you'd get the app broken up into tiny bits of grains of sand that could be poured, almost like a liquid, into any number of differently shaped containers.

The OS would then have fine-grained legibility over all of the sensitive behaviors of the app and how they could be combined.