In Blink’s LayoutNG engine, the FragmentBuilder class serves as a vital instrument for generating immutable fragment trees, which represent the final geometric output of the layout process. This system separates the mutable DOM from read-only layout results, facilitating more efficient downstream operations like painting, hit-testing, and accessibility. The builder acts as an assembly point, gathering data on logical offsets, sizes, and child fragments before converting them into a physical coordinate system. It also manages complex layout scenarios, including block fragmentation, floating elements, and out-of-flow positioned descendants. By producing reusable and cached fragments, the architecture enhances performance and ensures a clear distinction between layout inputs and rendered outputs. Recent updates have streamlined this process by unifying builder classes and introducing specialized helpers for inline text and line boxes.