Hit Testing Architecture in Blink and Chromium

This document explores the technical architecture of hit testing within the Chromium and Blink engines, the process used to link user input to specific interface elements. The system operates across multiple processes and threads, using the Viz compositor to route events correctly, especially when dealing with out-of-process iframes. While the Blink main thread performs the final, precise mapping using the layout tree and paint order, the compositor thread handles high-speed tasks like scrolling to minimize latency. Special mechanisms are also described for accessibility hit testing, which prioritizes immediate responsiveness through asynchronous tree updates. To maintain high performance, the engine employs caching strategies and coordinate translations that account for CSS transforms and device scaling. Overall, the text highlights how Chromium balances accuracy and speed by coordinating various rendering layers and hardware-accelerated components.