A whole bunch of changes

test/vehicle_detail_screen_test.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:provider/provider.dart';
+
+import 'package:fuel_tax_tracker/models/vehicle.dart';
+import 'package:fuel_tax_tracker/screens/vehicle_detail_screen.dart';
+import 'package:fuel_tax_tracker/services/app_state.dart';
+
+/// Covers the "modern navigation" swipe added to the single-vehicle
+/// receipt page: it's a PageView over every vehicle, starting on whichever
+/// one was tapped — sliding left/right moves to the next/previous vehicle,
+/// same as swiping between MainShell's bottom-nav tabs.
+void main() {
+  final vehicleA = Vehicle(id: 'a', vin: 'VINAAAAAAAAAAAAAA', nickname: 'Truck', updatedAt: DateTime.utc(2026, 1, 1));
+  final vehicleB = Vehicle(id: 'b', vin: 'VINBBBBBBBBBBBBBB', nickname: 'Van', updatedAt: DateTime.utc(2026, 1, 1));
+  final vehicleC = Vehicle(id: 'c', vin: 'VINCCCCCCCCCCCCCC', nickname: 'Sedan', updatedAt: DateTime.utc(2026, 1, 1));
+
+  Future<void> pumpDetail(WidgetTester tester, AppState appState, String vehicleId) async {
+    await tester.pumpWidget(
+      ChangeNotifierProvider.value(
+        value: appState,
+        child: MaterialApp(home: VehicleDetailScreen(vehicleId: vehicleId)),
+      ),
+    );
+  }
+
+  testWidgets('starts on the requested vehicle, not necessarily the first in the list',
+      (tester) async {
+    final appState = AppState()
+      ..isLoading = false
+      ..vehicles = [vehicleA, vehicleB, vehicleC];
+
+    await pumpDetail(tester, appState, 'b');
+
+    expect(find.widgetWithText(AppBar, 'Van'), findsOneWidget);
+  });
+
+  testWidgets('a right-to-left swipe advances to the next vehicle', (tester) async {
+    final appState = AppState()
+      ..isLoading = false
+      ..vehicles = [vehicleA, vehicleB, vehicleC];
+
+    await pumpDetail(tester, appState, 'a');
+    expect(find.widgetWithText(AppBar, 'Truck'), findsOneWidget);
+
+    await tester.fling(find.byType(PageView), const Offset(-400, 0), 1000);
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(AppBar, 'Van'), findsOneWidget);
+  });
+
+  testWidgets('a left-to-right swipe goes back to the previous vehicle', (tester) async {
+    final appState = AppState()
+      ..isLoading = false
+      ..vehicles = [vehicleA, vehicleB, vehicleC];
+
+    await pumpDetail(tester, appState, 'c');
+    expect(find.widgetWithText(AppBar, 'Sedan'), findsOneWidget);
+
+    await tester.fling(find.byType(PageView), const Offset(400, 0), 1000);
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(AppBar, 'Van'), findsOneWidget);
+  });
+
+  testWidgets('swiping does not go past the last vehicle', (tester) async {
+    final appState = AppState()
+      ..isLoading = false
+      ..vehicles = [vehicleA, vehicleB];
+
+    await pumpDetail(tester, appState, 'b');
+
+    await tester.fling(find.byType(PageView), const Offset(-400, 0), 1000);
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(AppBar, 'Van'), findsOneWidget, reason: 'stayed on the last page');
+  });
+
+  testWidgets('a single vehicle has nothing to swipe to and just shows itself', (tester) async {
+    final appState = AppState()
+      ..isLoading = false
+      ..vehicles = [vehicleA];
+
+    await pumpDetail(tester, appState, 'a');
+
+    expect(find.widgetWithText(AppBar, 'Truck'), findsOneWidget);
+  });
+}