Function set_screen_layout 

pub fn set_screen_layout(
    socket_path: &str,
    rects: Vec<(i32, i32, i32, i32)>,
) -> Result<usize, String>

Lay the session’s screens out at rects, one (x, y, width, height) per screen in screen order, so its own layout matches the arrangement the capture outputs were placed in.

A session compositor arranges the screens it opens by its own rule — wlroots puts them side by side in the order they appear — and that layout, not the capture one, is what places windows and carries the pointer between screens. Every screen is positioned in one configuration: applied one at a time, an intermediate state overlaps two screens and the compositor reflows around it. The rectangles are capture pixels; the positions land in the session’s logical space, where a scaled screen is its mode over its scale, closed up so a screen beside a scaled one meets its logical edge rather than its pixel edge.

Returns how many screens were positioned; 0 when the compositor manages no outputs for clients, which is where a session on KWin lands (it offers kde_output_management_v2 instead) and where the arrangement stays whatever that compositor chose.