This mode selects `bwrap` outside of Flatpaks and usually `flatpak-spawn` inside of Flatpaks.
The sandbox is disabled automatically inside of Flatpak development environments. Inside of Flatpaks, `flatpak-spawn` is used to create the sandbox. This mechanism starts an installed Flatpak with the same app id. For development, Flatpak are usually not installed and the sandbox can therefore not be used. If the sandbox has been started via `flatpak-builder --run` (i.e. without installed Flatpak) and the app id ends with `Devel`, the sandbox is disabled.