Function first_dangling_leaf_id 

fn first_dangling_leaf_id(
    node: &LayoutNode,
    live_block_ids: &HashSet<String>,
) -> Option<String>

First leaf (depth-first) whose data.block_id is set but not present in live_block_ids. data.block_id is only ever non-empty on leaves — container/group nodes have data: None (see ensure_group_node).