InputHandlerProxy in Blink and cc: A Technical Report

The provided technical report details the architecture and function of the InputHandlerProxy within the Chromium renderer as of early 2026. This component operates on the compositor thread to manage user inputs like scrolling and touch gestures, serving as a vital link between the Blink rendering engine and the cc compositor. By handling these events off the main thread, it ensures scrolling remains smooth even when the main thread is occupied with complex tasks. The sources describe core responsibilities such as event coalescing, latency reduction via scroll prediction, and managing main-thread hit tests when the compositor cannot identify a scroll target. Additionally, the report outlines the Mojo IPC interfaces used for browser-to-renderer communication and highlights the historical migration of this code into the Blink platform. Detailed sequence diagrams and code maps further illustrate how the proxy coordinates complex interactions like scrollbar dragging and VSync-aligned event delivery.