A feed that jitters on device but “looks fine” in Simulator is often observation, not layout. I traced a scroll hitch to a single @Observable retained by two ancestors—body executions fanned out and taxed frames.

• Give each @Observable exactly one owner and pass references downward. Don’t cache or re-wrap the same instance in multiple ancestors.
• Treat every View.body read as a subscription; narrow reads and move derived work into mutators so invalidations stay local.
• Instrument mutations with OSSignposter, profile on device with Time Profiler, and correlate mutate → render waves. 🔬
• Gate migrations behind a provider façade so you can swap implementations and roll back without touching call sites.
• Add render-count tests for hot paths (scroll, type-ahead); assert idempotence so no-op state sets don’t re-render.

Choose @Observable when your ownership graph is single-writer and explicit; keep MVVM façades where UIKit or cross-platform surfaces remain, and flip via a feature flag.

What guard