Can you be more precise about what scenario you are envisioning?
You cannot cover the entire screen if the entire screen may act as a camera.
You can cover/disable the dedicated camera.
What do you mean by asking about a "scenario"?
I think Cider9986's comment [1] on the OP covers it pretty well.
[1]: https://news.ycombinator.com/item?id=48700061
But first, could you name two of the many utilitarian purposes?
You cannot cover the entire screen if the entire screen may act as a camera.
You can cover/disable the dedicated camera.
What do you mean by asking about a "scenario"?