spinner feedback to ui 2

lib/utils/deferred_tab_work.dart

@@ -0,0 +1,50 @@
+/// Schedule heavy tab work off the first paint, yielding so nav stays responsive.
+library;
+
+import 'dart:async';
+
+/// Runs [compute] after a frame, cancellable via [cancel] / generation bumps.
+class DeferredTabWork<T> {
+  int _gen = 0;
+  Object? _loadingKey;
+  T? value;
+  Object? valueKey;
+
+  bool readyFor(Object key) => value != null && valueKey == key;
+
+  bool get isLoading => _loadingKey != null;
+
+  void cancel() {
+    _gen++;
+    _loadingKey = null;
+  }
+
+  /// Starts work for [key] unless already ready or in flight for that key.
+  void ensure({
+    required Object key,
+    required Future<T> Function(Future<void> Function() yieldUi) compute,
+    required void Function() onUpdate,
+    required bool Function() mounted,
+  }) {
+    if (readyFor(key) || _loadingKey == key) return;
+    _loadingKey = key;
+    final gen = ++_gen;
+
+    unawaited(() async {
+      await Future<void>.delayed(Duration.zero);
+      if (!mounted() || gen != _gen) return;
+
+      Future<void> yieldUi() async {
+        await Future<void>.delayed(Duration.zero);
+      }
+
+      final result = await compute(yieldUi);
+      if (!mounted() || gen != _gen) return;
+
+      value = result;
+      valueKey = key;
+      if (_loadingKey == key) _loadingKey = null;
+      onUpdate();
+    }());
+  }
+}