Why privacy screen apps black out your whole screen
Because blacking out the screen is the only thing an Android app can reliably do. An app can draw an overlay on top of other apps; it cannot change how the panel emits light. So it cannot make your screen dark from the side and bright from the front — only dark for everyone, including you. Most apps in this category took the shortest path to that: watch the accelerometer, and when the phone tilts past a threshold, cover everything.
What the platform actually allows
Android exposes one relevant capability: a permission to draw over other apps. An app with it can put a layer of any colour and opacity on top of your screen, and that is the whole toolkit. Viewing angle is a property of the display stack — the polarisers, the backlight, the pixel structure — and no permission grants access to it. A privacy film works because it is a physical filter sitting in the light path. Software is on the wrong side of the glass.
Why tilt detection became the standard design
Given only an overlay, the obvious product is: assume that if the phone is tilted, someone else is looking, and go black. It demos well in a thirty-second video. It is also a blunt rule applied to a noisy sensor, and phones tilt constantly — walking, sitting down, reaching for a handrail, lying in bed.
What people report after installing them
The reviews on apps built this way repeat the same two complaints, paraphrased: "the slightest tilt turns the whole screen black, so I had to turn it off", and "if I can't see it either, what is the point?" Both are describing the same design flaw. A privacy feature that also blocks the owner is not a privacy feature, it is an off switch with extra steps — so people uninstall it and go back to nothing.
The alternative: cover everything, open a window
The useful version of an overlay is not all-or-nothing. Cover the screen, then open a small readable window over the line you are actually reading, and let that window move with you. A glance from the next seat lands on a covered screen and catches a fragment instead of the whole message. You keep reading at full brightness. Nothing is triggered by a sensor guessing at your intent — you decide when it is on.
Be clear about the ceiling: this reduces how much is exposed, not the angle it is exposed at. That is the honest limit of any software approach, and an app that claims more is describing hardware.
A warning about the ads
You will see video ads for privacy apps showing a screen that looks perfect head-on and goes black from the side. That footage is of a phone with a built-in privacy display — a hardware feature of certain recent models. An app cannot reproduce it on your phone. If an ad promises that effect from an installable app, it is selling you the blackout described above.
Peekless
Peekless is the window approach. It covers the screen and opens a movable, resizable reading window where you are looking, so your phone stays usable while the rest is hidden. No tilt guessing, no full blackout — you turn it on and off yourself, from the notification shade. It runs offline and asks only for the overlay permission it needs to draw.