Function compute_in_flow_root_layout 

fn compute_in_flow_root_layout(
    tree: &mut (impl LayoutContainingBlock + CacheTree),
    root: NodeId,
    available_space: Size<AvailableSpace>,
    root_is_cached: &mut bool,
) -> (Layout, OofCandidates)

Lay out an in-flow (or position: relative) root node against the available space, store its layout and return it along with the out-of-flow candidates bubbled up from its subtree.