Layout takes the styled DOM and produces a fragment tree of positioned boxes. In modern Blink this is LayoutNG; the core artifact is NGPhysicalFragment, which stores geometry and is used downstream for pre‑paint (e.g., to compute clips, overflow, and hit regions). (Chromium Git Repositories)

Representative APIs
Geometry/ink overflow helpers and accessors on NGPhysicalFragment expose the post‑layout shape of the page that pre‑paint/paint will consume. (See header for geometry utilities.) (Chromium Git Repositories)

Hands off to: PrePaintTreeWalk (to build/refresh property trees and invalidations over the laid‑out fragments).