Function pick_handle 

pub fn pick_handle(
    mode: GizmoMode,
    pivot: Vec3,
    basis: &[Vec3; 3],
    size: f32,
    ray: &Ray,
) -> Option<GizmoAxis>

The nearest handle whose geometry the ray passes within tolerance of, or None when the cursor is not over the manipulator.

Nearest along the ray, so a handle in front wins over one behind it — which is what “the one you can see” means when two overlap on screen.