Function show_windows 

pub(crate) fn show_windows(
    connection: &Connection,
    queue: &mut EventQueue<Probe>,
    probe: &mut Probe,
    seconds: u64,
)

Map two windows – one decorated by the compositor, one not – and hold them up to be looked at.

WL_PROBE_WINDOWS=<seconds> holds them; WL_PROBE_FULLSCREEN=1 makes the first one ask for the screen a second in, which is what makes the transition photographable.