Function select_audience_output 

pub fn select_audience_output(
    monitors: &[MonitorInfo],
    target: FullscreenTarget,
    presenter_output: Option<u32>,
) -> Option<u32>

Pick the audience display for a two-screen presentation. None means there is no second output, so the caller keeps its single-window fallback.

The presenter’s current output does not disqualify a display: a compositor that pairs the audience surface moves the presenter view to select_presenter_output, the way a laptop plugged into a television is expected to behave. It only breaks ties, so of two identical screens the one the presenter is not using gets the slides.