Function detect 

fn detect() -> Session

Pick the splash backend by centering ability. The X11 backend self-centers (override-redirect, positioned on the RANDR primary) and XWayland honors that, so prefer it whenever an X server is actually reachable — including under a native-Wayland session, where the Wayland backend can’t self-center and Mutter drops the splash top-left. Only with no usable X display do we fall back to the best-effort Wayland backend.

An explicit AGENTMUX_OZONE_PLATFORM=x11|wayland still pins a specific backend (escape hatch / for exercising the Wayland path itself); note that pinning wayland re-introduces the top-left placement Mutter gives uncentered toplevels.