Function single_leaf_tree 

pub(crate) fn single_leaf_tree(
    block_id: &str,
) -> (LayoutNode, String, Vec<LeafOrderEntry>)

Pure builder for a single-pane tree wrapping exactly one block — no splits. Used by the post-bootstrap CreateWindow seed (handle_create_window) when the caller supplies a specific seed_view (e.g. “Open in New Window” on a widget) instead of wanting the default 3-pane launch layout — the new window’s sole pane is the requested widget, not that widget plus the default agent/swarm/sysinfo split.