Function create_view_on 

pub(crate) fn create_view_on(
    state: &mut AppState,
    id: u32,
    owner: u32,
    x: i32,
    y: i32,
    width: i32,
    height: i32,
) -> bool

Add a view: a display over a rectangle of owner’s output.

A view renders and encodes on its own – its own damage tracker, frame buffer, and capture – but publishes no wl_output, so the session sees one screen however many displays the client is shown. That is what a window drag needs: the pointer grab it runs under stays inside one surface across the whole desk, where separate screens stop it at the edge of the one it began on.