An app that outlines the curvature of the Earth over the horizon in the phone's camera view. People can zoom out/in to see how their view fits on the global scale and hopefully understand why it seems flat to them.
This Reddit post demonstrates that even on a high altitude flight people could see what they choose/expect (flat Earth) rather than what you try to show them (slight curvature of the globe). This app gives them the ability to explore other possibilities on their own terms. Hopefully, it helps some people.
A person who believes the Earth is flat flies to a place far enough to require a high-altitude flight. Upon reaching 10km (35000 ft) cruising altitude, they point their phone's camera at the horizon. The app knows the approximate area that is in view based on the phone's altitude (GPS) and the camera's capabilities when facing the unobstructed horizon.
The person then gets to play by zooming in and out, to realize how tiny their view is compared to the globe.
The app should probably also teach people about gravity, why we don't fall off, why up always feels up, etc. To make this intuitively understandable, what features should be added?