Blink's PendingLayer: Bridge Between Paint and Compositor

The document provides an extensive technical overview of the PendingLayer class, a critical internal component within Blink’s Composite After Paint (CAP) architecture. PendingLayer acts as a liaison, aggregating multiple Paint Chunks (painted content units) into a single logical unit that will eventually become a single cc::Layer in the Chromium compositor. The text details the class's structure, including fields for geometry, content references, and Compositing Type, and describes its transient lifecycle during the frame update process led by the PaintArtifactCompositor. Crucially, the document explains the complex layerization heuristics—including property tree compatibility, upcasting, overlap checks, and LCD text preservation—used to minimize the total layer count for improved performance while maintaining visual correctness. Finally, it outlines how this mechanism supports modern optimizations, such as partial repaint fast-paths and scroll unification, by providing stable layer structures across frames.