cc::ClipTree

The source provides an extensive technical overview of the ClipTree within Chromium's compositor property tree architecture, explaining its critical role in managing geometric clipping. The ClipTree is composed of ClipNodes, which represent specific clipping boundaries (like CSS overflow:hidden) and decouple the clipping hierarchy from the layer hierarchy. Central to its function is the computation of the combined clip, which is the intersection of a node's local clip and its ancestors' clips, often requiring coordinate transformation via the TransformTree. The document details how ClipNodes handle special cases, such as clip expansion for visual effects like blurs, and clip resets when a new render surface isolates content from ancestor clips. Finally, the text describes the performance-critical process of building, incrementally updating, and using the ClipTree data during quad generation for culling and setting GPU scissor regions.