The source provides a highly technical, in-depth analysis of the Chromium browser's navigation process under the modern PlzNavigate architecture, focusing specifically on the NavigationRequest class. This central object, which operates entirely on the UI thread, orchestrates every top-level navigation, tracking it from initiation (typing a URL or clicking a link) through execution of security policies like CSP and COEP, handling redirects and throttles, and coordinating with the network stack via NavigationURLLoader. The text extensively details the lifecycle of NavigationRequest, including its creation, the complex sequence of steps before starting a network request (such as running beforeunload handlers), the final decision-making regarding which RenderFrameHost will commit the document (often involving process swaps for Site Isolation), and its ultimate cleanup and destruction upon successful commit or failure. The explanation also outlines how NavigationRequest interacts with critical browser components like NavigationHandle (for observers) and NavigationController (for session history), functioning as the essential "glue" between the network, security policies, and the rendering engine.
