Function plan 

pub fn plan(
    offers: &[ScrollOffer],
    roots: &[&dyn Widget],
    scale: f64,
    size: (u32, u32),
) -> CopyPlan

Plan offers against the trees under roots just drawn (the app’s, then any the window draws beside it, such as History’s toolbar), at scale device pixels per logical pixel, for a width × height target: each becomes a copy and the damage around it, or, when it cannot be copied exactly, damage over its whole viewport.