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This is why you want a trusted secure hardware boundary.

The Apple Pay payment flow on iOS/watchOS requires you to double press the power/side button to authorize a transaction, which userland apps can’t intercept.

Similarly the camera activity indicator on iOS appears somewhere on the screen that regular apps can’t display pixels to (the Dynamic Island, on most recent iPhones)

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Technically you don't actually need an area that only the OS can exclusively access. If the OS is allowed to draw on top of everything you can still solve this problem, as Apple does on older iPhones. (Though you still need to be very careful, as markings displayed out-of-context can be made to be misleading. An orange dot disappears in an orange underlay, for example.)

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