Function find_leaf_containing_block 

pub fn find_leaf_containing_block<'a>(
    tree: &'a LayoutNode,
    block_id: &str,
) -> Option<&'a LayoutNode>

Find the (first) leaf that has block_id as its visible block OR as any member of its block_stack. Unlike [find_node_id_by_block], which only matches the visible block, this finds the pane a background tab lives in. Returns the leaf node itself.