Runtime-level SwiftUI guidance for macOS apps. Covers observation internals, concurrency scheduling, view identity, performance, AppKit bridging, SwiftData, and accessibility — with deep explanations of why patterns work, not just what to do. Built from developing a production macOS browser with SwiftUI and from reading the Swift runtime source. Targets macOS 26+, Swift 6.2 with strict concurrency. Designed for frontier models: skips basics, focuses on non-obvious traps and mechanisms that even experienced developers get wrong.