The provided sources describe the fragment tree spine in Blink, which is the architectural "backbone" of cached ancestor fragments that must be updated following a localized subtree relayout. Because Blink’s modern LayoutNG framework treats the fragment tree as an immutable output used for painting and hit-testing, isolated changes to a descendant subtree leave older ancestor fragments pointing at stale data. To resolve this, the spine repair mechanism climbs the containing box chain, shallow-cloning each cached LayoutResult and PhysicalBoxFragment to re-stitch them to the latest post-layout children. This process ensures structural consistency throughout the fragment tree without requiring a full, expensive relayout of the entire document. The system is tightly integrated with relayout boundaries, fragmentation rules, and pre-paint invalidation to maintain performance and safety during incremental updates. Ultimately, the spine serves as the essential link that reconciles localized layout changes with a durable, read-only rendering hierarchy.