I have never taken a selfie in my entire life. The feature is entirely pointless for me. Are there any smartphones that don't have a front facing camera?
Painting a small black dot over the front facing camera caused me issues on my previous phone, so I suspect some cameras cheap out on the proximity sensor and use the camera instead.
You're thinking of the gyroscope, but no. Some phones have a dedicated proximity sensor for things like detecting if you're in a call and have the phone up to your ear, but more and more phones (especially the budget ones) these days use a "virtual" proximity sensor which utilizes the front camera to perform the same function.
that's interesting... wouldn't that kind of sensor consume a lot of battery? turning the camera on and off every time you use it, and to determine the distance I assume it's autofocus?