spinner feedback to ui

lib/screens/look_ahead_screen.dart

@@ -1,6 +1,8 @@
 /// Look ahead: projected balance from recurring templates through a date.
 library;
 
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
@@ -16,6 +18,7 @@ import '../utils/look_ahead.dart';
 import '../utils/navigation.dart';
 import '../utils/tx_calendar.dart';
 import '../widgets/empty_state.dart';
+import '../widgets/loading_view.dart';
 import '../widgets/recurring_template_sheet.dart';
 import '../widgets/responsive_body.dart';
 import '../widgets/shell_tab_scope.dart';
@@ -23,6 +26,13 @@ import '../widgets/skip_build_when_covered.dart';
 
 enum _PeriodMode { monthly, yearly }
 
+class _ItemsSnapshot {
+  const _ItemsSnapshot({required this.key, required this.items});
+
+  final int key;
+  final List<RecurringItem> items;
+}
+
 class LookAheadScreen extends StatefulWidget {
   const LookAheadScreen({super.key});
 
@@ -35,8 +45,9 @@ class LookAheadScreen extends StatefulWidget {
 class _LookAheadScreenState extends State<LookAheadScreen> {
   var _period = _PeriodMode.monthly;
   late DateTime _cursor;
-  List<RecurringItem>? _cachedItems;
-  int? _itemsCacheKey;
+  _ItemsSnapshot? _snapshot;
+  int? _loadingKey;
+  int _computeGen = 0;
 
   @override
   void initState() {
@@ -129,9 +140,78 @@ class _LookAheadScreenState extends State<LookAheadScreen> {
     });
   }
 
+  int _itemsCacheKey(TxMetaController meta, BankController bank) =>
+      Object.hash(meta.storeRevision, identityHashCode(bank.transactions));
+
+  /// Resolve recurring amounts off the build path, yielding so nav stays live.
+  void _ensureItemsLoaded({
+    required int cacheKey,
+    required TxMetaController meta,
+    required BankController bank,
+  }) {
+    if (_snapshot?.key == cacheKey || _loadingKey == cacheKey) return;
+    _loadingKey = cacheKey;
+    final gen = ++_computeGen;
+    final templates = List<RecurringTemplate>.of(meta.recurringTemplates);
+    final transactions = bank.transactions;
+
+    unawaited(() async {
+      // Let the spinner / tab switch paint before heavy work.
+      await Future<void>.delayed(Duration.zero);
+      if (!mounted || gen != _computeGen) return;
+
+      final items = <RecurringItem>[];
+      for (var i = 0; i < templates.length; i++) {
+        final tpl = templates[i];
+        items.add(
+          RecurringItem(
+            templateId: tpl.id,
+            description: tpl.name,
+            amount: resolveRecurringAmount(
+              template: tpl,
+              transactions: transactions,
+              isInternalTransfer: (tx) => meta.metaFor(tx).internalTransfer,
+              labelIdsFor: meta.labelIdsFor,
+            ),
+            currency: tpl.currency,
+            dayOfMonth: tpl.dayOfMonth.clamp(1, 28),
+            ruleSummary: recurringRuleSummary(
+              template: tpl,
+              labelName: (id) => meta.labelById(id)?.name ?? 'label',
+            ),
+            sourceTxId: tpl.sourceTxId,
+            labelIds: [
+              if (tpl.labelId != null && tpl.labelId!.isNotEmpty) tpl.labelId!,
+            ],
+          ),
+        );
+        // Yield often so bottom-nav taps are not starved.
+        if (i % 2 == 1) {
+          await Future<void>.delayed(Duration.zero);
+          if (!mounted || gen != _computeGen) return;
+        }
+      }
+
+      if (!mounted || gen != _computeGen) return;
+      setState(() {
+        _snapshot = _ItemsSnapshot(key: cacheKey, items: items);
+        if (_loadingKey == cacheKey) _loadingKey = null;
+      });
+    }());
+  }
+
   @override
   Widget build(BuildContext context) {
-    if (ShellTabScope.of(context) != LookAheadScreen.tabIndex) {
+    final active = ShellTabScope.of(context) == LookAheadScreen.tabIndex;
+    if (!active) {
+      if (_loadingKey != null) {
+        WidgetsBinding.instance.addPostFrameCallback((_) {
+          if (!mounted) return;
+          if (ShellTabScope.of(context) == LookAheadScreen.tabIndex) return;
+          _computeGen++;
+          _loadingKey = null;
+        });
+      }
       return const SizedBox.expand();
     }
     return SkipBuildWhenCovered(
@@ -165,22 +245,29 @@ class _LookAheadScreenState extends State<LookAheadScreen> {
     final currentAmount = balance?.amount ?? 0;
     final today = _today;
     final through = _through;
-    final cacheKey = Object.hash(
-      meta.storeRevision,
-      identityHashCode(bank.transactions),
-    );
-    if (_cachedItems == null || _itemsCacheKey != cacheKey) {
-      _itemsCacheKey = cacheKey;
-      _cachedItems = recurringItemsFromTemplates(
-        templates: meta.recurringTemplates,
-        transactions: bank.transactions,
-        isInternalTransfer: (tx) => meta.metaFor(tx).internalTransfer,
-        labelIdsFor: meta.labelIdsFor,
-        labelName: (id) => meta.labelById(id)?.name ?? 'label',
+    final cacheKey = _itemsCacheKey(meta, bank);
+    if (_snapshot?.key != cacheKey && _loadingKey != cacheKey) {
+      WidgetsBinding.instance.addPostFrameCallback((_) {
+        if (!mounted) return;
+        if (ShellTabScope.of(context) != LookAheadScreen.tabIndex) return;
+        _ensureItemsLoaded(cacheKey: cacheKey, meta: meta, bank: bank);
+      });
+    }
+
+    final snapshot = _snapshot;
+    final ready = snapshot != null && snapshot.key == cacheKey;
+
+    if (!ready) {
+      return Scaffold(
+        appBar: AppBar(
+          title: const Text('Look ahead'),
+          actions: bankAppBarActions(context: context),
+        ),
+        body: const LoadingView(message: 'Calculating look ahead…'),
       );
     }
-    final items = _cachedItems!;
 
+    final items = snapshot.items;
     final projected = projectedOccurrences(
       items: items,
       from: today,