flutter-performancelisted

# Flutter Performance Hold a steady 60 fps (<=16 ms/frame; <=8 ms on 120 Hz) on a floor device (low-end Android / older iPhone). Performance is a property you **measure in profile mode**, not one you assert. It comes from rebuilding less, painting cheaply, and keeping both the UI thread and the raster thread free. ## Non-negotiable rules 1. **`const` everything legal.** A `const` subtree is skipped on rebuild — the framework short-circuits it by identity. Keep `prefer_const_constructors` on as an error; a non-const literal that could be const is a lint failure, not a preference. 2. **Shrink the rebuild scope to the smallest changing widget.** Never `ref.watch(provider)` for a whole state object when one field changed — watch `provider.select((s) => s.field)`. A HUD counter tick must never rebuild a heavy sibling. Rebuild scope is the single biggest lever on build-thread cost. 3. **Lazy everything long.** `ListView.builder` / `GridView.builder` / slivers for variable or unbounded content. `ListView(children: items.map(...).toList())` builds every off-screen row up front — refuse it for anything not tiny and fixed. 4. **No expensive work in `build()`.** No `jsonDecode`, sort, regex, `DateTime` math, file/network, or large allocation — `build()` may run every frame. Compute in the Notifier/ViewModel once and cache the result in immutable state. 5. **Heavy CPU off the UI isolate.** Parse large payloads, process images, crunch numbers in `compute()` or a spawned `Isolate`. Bloc