Fix critical bugs: Level write-back, null safety, UX improvements

Critical bug fixes:
- Level now written back in PreparePKM (was causing data loss)
- Undo/Redo null safety on GetBoxSlotAtIndex result
- SKSurface.Create null check in UpdateLegality

UX improvements:
- Unsaved changes confirmation on window close
- Box mouse wheel navigation (scroll to change box)
- Tooltips: PID hex info, Species number, Nature stat effects

PKHeX.Avalonia/ViewModels/MainWindowViewModel.cs

@@ -51,6 +51,12 @@ public partial class MainWindowViewModel : ObservableObject
     [ObservableProperty]
     private string _statusMessage = "Ready. Open a save file to begin.";
 
+    /// <summary>
+    /// Indicates whether the currently loaded save file has been modified since it was last saved or loaded.
+    /// </summary>
+    [ObservableProperty]
+    private bool _hasUnsavedChanges;
+
     /// <summary>
     /// The currently active UI language code, matching <see cref="GameLanguage"/> codes.
     /// </summary>
@@ -216,6 +222,7 @@ private async Task SaveFileAsync()
             CreateAutoBackup(path);
 
             ExportSAV(SaveFile, path);
+            HasUnsavedChanges = false;
             StatusMessage = $"Saved to {Path.GetFileName(path)}";
         }
         catch (Exception ex)
@@ -365,6 +372,7 @@ private void LoadSaveFile(SaveFile sav, string path)
     {
         SaveFile = sav;
         HasSaveFile = true;
+        HasUnsavedChanges = true;
         _loadedFilePath = path;
 
         SpriteUtil.Initialize(sav);

PKHeX.Avalonia/ViewModels/PKMEditorViewModel.cs

@@ -62,6 +62,27 @@ public partial class PKMEditorViewModel : ObservableObject
             UpdateSprite();
     }
 
+    /// <summary>Tooltip showing the numeric species ID.</summary>
+    public string SpeciesTooltip => Entity is null ? "" : $"Species #{Entity.Species:000}";
+
+    /// <summary>
+    /// Tooltip showing which stats are raised/lowered by the current nature.
+    /// </summary>
+    public string NatureTooltip
+    {
+        get
+        {
+            var n = Nature;
+            var idx = (int)n;
+            if ((uint)idx >= 25) return n.ToString();
+            var up = idx / 5;
+            var down = idx % 5;
+            if (up == down) return $"{n} (Neutral)";
+            var statNames = new[] { "Atk", "Def", "Spe", "SpA", "SpD" };
+            return $"{n} (+{statNames[up]} / -{statNames[down]})";
+        }
+    }
+
     // Stat Nature (Gen 8+)
     [ObservableProperty] private Nature _statNature;
     [ObservableProperty] private bool _hasStatNature;
@@ -656,6 +677,7 @@ private void UpdateRegionNames()
                 try { Nickname = speciesName; }
                 finally { _isPopulating = false; }
             }
+            OnPropertyChanged(nameof(SpeciesTooltip));
             UpdateSprite();
             UpdateLegality();
         }
@@ -672,6 +694,7 @@ private void UpdateRegionNames()
             OnPropertyChanged(nameof(SpAColor));
             OnPropertyChanged(nameof(SpDColor));
             OnPropertyChanged(nameof(SpeColor));
+            OnPropertyChanged(nameof(NatureTooltip));
             RecalcStats();
             UpdateLegality();
         }
@@ -1545,6 +1568,7 @@ public void PopulateFields(PKM pk)
         if (uint.TryParse(PidHex, System.Globalization.NumberStyles.HexNumber, null, out var pid))
             Entity.PID = pid;
         Entity.EXP = Exp;
+        Entity.Stat_Level = Level;
         Entity.CurrentFriendship = (byte)Math.Clamp(Friendship, 0, 255);
         Entity.Language = Language;
 
@@ -2268,6 +2292,7 @@ private void UpdateLegality()
             var color = valid ? SKColors.Green : SKColors.Red;
 
             using var surface = SKSurface.Create(new SKImageInfo(24, 24));
+            if (surface is null) { LegalityImage = null; return; }
             var canvas = surface.Canvas;
             canvas.Clear(SKColors.Transparent);
             using var paint = new SKPaint { Color = color, IsAntialias = true };

PKHeX.Avalonia/ViewModels/SAVEditorViewModel.cs

@@ -64,6 +64,7 @@ public void Undo()
         var current = change.IsParty
             ? _sav.GetPartySlotAtIndex(change.Slot)
             : _sav.GetBoxSlotAtIndex(change.Box, change.Slot);
+        if (current is null) return;
         _redoStack.Push(new SlotChange(change.Box, change.Slot, current.DecryptedBoxData, change.IsParty));
 
         // Restore old state
@@ -92,6 +93,7 @@ public void Redo()
         var current = change.IsParty
             ? _sav.GetPartySlotAtIndex(change.Slot)
             : _sav.GetBoxSlotAtIndex(change.Box, change.Slot);
+        if (current is null) return;
         _undoStack.Push(new SlotChange(change.Box, change.Slot, current.DecryptedBoxData, change.IsParty));
 
         // Restore redo state

PKHeX.Avalonia/Views/MainWindow.axaml.cs

@@ -3,6 +3,9 @@
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Input;
+using Avalonia.Interactivity;
+using Avalonia.Layout;
+using Avalonia.Media;
 using Avalonia.Styling;
 using PKHeX.Avalonia.ViewModels;
 
@@ -10,12 +13,72 @@ namespace PKHeX.Avalonia.Views;
 
 public partial class MainWindow : Window
 {
+    private bool _forceClose;
+
     public MainWindow()
     {
         InitializeComponent();
 
         AddHandler(DragDrop.DropEvent, OnDrop);
         AddHandler(DragDrop.DragOverEvent, OnDragOver);
+        Closing += OnWindowClosing;
+    }
+
+    private async void OnWindowClosing(object? sender, WindowClosingEventArgs e)
+    {
+        if (_forceClose)
+            return;
+
+        if (DataContext is MainWindowViewModel vm && vm.HasUnsavedChanges)
+        {
+            e.Cancel = true;
+
+            var confirmDialog = new Window
+            {
+                Title = "Unsaved Changes",
+                Width = 360,
+                Height = 140,
+                CanResize = false,
+                WindowStartupLocation = WindowStartupLocation.CenterOwner,
+            };
+
+            var result = false;
+
+            var cancelBtn = new Button { Content = "Cancel", Width = 80 };
+            cancelBtn.Click += (_, _) => { result = false; confirmDialog.Close(); };
+
+            var closeBtn = new Button { Content = "Close Anyway", Width = 100 };
+            closeBtn.Click += (_, _) => { result = true; confirmDialog.Close(); };
+
+            confirmDialog.Content = new StackPanel
+            {
+                Margin = new Thickness(20),
+                Spacing = 16,
+                Children =
+                {
+                    new TextBlock
+                    {
+                        Text = "You have unsaved changes. Close without saving?",
+                        TextWrapping = TextWrapping.Wrap,
+                    },
+                    new StackPanel
+                    {
+                        Orientation = Orientation.Horizontal,
+                        HorizontalAlignment = HorizontalAlignment.Right,
+                        Spacing = 8,
+                        Children = { cancelBtn, closeBtn },
+                    },
+                },
+            };
+
+            await confirmDialog.ShowDialog(this);
+
+            if (result)
+            {
+                _forceClose = true;
+                Close();
+            }
+        }
     }
 
     private void OnDragOver(object? sender, DragEventArgs e)

PKHeX.Avalonia/Views/PKMEditorView.axaml

@@ -61,7 +61,7 @@
                     <!-- PID -->
                     <TextBlock Grid.Row="0" Grid.Column="0" Text="PID:" TextAlignment="Right" VerticalAlignment="Center" Margin="0,0,6,0" />
                     <StackPanel Grid.Row="0" Grid.Column="1" Orientation="Horizontal" VerticalAlignment="Center">
-                        <TextBox Text="{Binding PidHex}" Height="25" Width="72" FontFamily="Courier New,Consolas,monospace" />
+                        <TextBox Text="{Binding PidHex}" Height="25" Width="72" FontFamily="Courier New,Consolas,monospace" ToolTip.Tip="Personality ID (hex)" />
                         <Button Content="&#x2606;" Command="{Binding ShinytizeCommand}" Width="24" Height="25" Padding="0" FontSize="14" Margin="4,0,0,0" ToolTip.Tip="Toggle Shiny" />
                         <Button Content="Reroll" Command="{Binding RerollPidCommand}" Height="25" Padding="4,0" FontSize="9" Margin="2,0,0,0" />
                     </StackPanel>
@@ -70,7 +70,8 @@
                     <TextBlock Grid.Row="1" Grid.Column="0" Text="Species:" TextAlignment="Right" VerticalAlignment="Center" Margin="0,0,6,0" />
                     <StackPanel Grid.Row="1" Grid.Column="1" Orientation="Horizontal" VerticalAlignment="Center">
                         <ComboBox ItemsSource="{Binding SpeciesList}" SelectedItem="{Binding SelectedSpecies}"
-                                  DisplayMemberBinding="{Binding Text, DataType=core:ComboItem}" Height="25" Width="144" />
+                                  DisplayMemberBinding="{Binding Text, DataType=core:ComboItem}" Height="25" Width="144"
+                                  ToolTip.Tip="{Binding SpeciesTooltip}" />
                         <TextBlock Text="{Binding GenderSymbol}" FontSize="14" FontWeight="Bold"
                                    VerticalAlignment="Center" Margin="6,0,0,0" Width="16" />
                     </StackPanel>
@@ -96,7 +97,8 @@
                     <!-- Nature -->
                     <TextBlock Grid.Row="4" Grid.Column="0" Text="Nature:" TextAlignment="Right" VerticalAlignment="Center" Margin="0,0,6,0" />
                     <ComboBox Grid.Row="4" Grid.Column="1" ItemsSource="{Binding NatureList}" SelectedItem="{Binding SelectedNature}"
-                              DisplayMemberBinding="{Binding Text, DataType=core:ComboItem}" Height="25" Width="144" HorizontalAlignment="Left" />
+                              DisplayMemberBinding="{Binding Text, DataType=core:ComboItem}" Height="25" Width="144" HorizontalAlignment="Left"
+                              ToolTip.Tip="{Binding NatureTooltip}" />
 
                     <!-- Stat Nature (Gen 8+) -->
                     <TextBlock Grid.Row="5" Grid.Column="0" Text="Stat Nature:" TextAlignment="Right" VerticalAlignment="Center" Margin="0,0,6,0"

PKHeX.Avalonia/Views/SAVEditorView.axaml.cs

@@ -1,4 +1,7 @@
 using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Interactivity;
+using PKHeX.Avalonia.ViewModels;
 
 namespace PKHeX.Avalonia.Views;
 
@@ -7,5 +10,20 @@ public partial class SAVEditorView : UserControl
     public SAVEditorView()
     {
         InitializeComponent();
+
+        // Tunnel handler so we intercept wheel events on the box area for box navigation
+        AddHandler(PointerWheelChangedEvent, OnBoxWheel, RoutingStrategies.Tunnel);
+    }
+
+    private void OnBoxWheel(object? sender, PointerWheelEventArgs e)
+    {
+        if (DataContext is SAVEditorViewModel vm && vm.IsLoaded)
+        {
+            if (e.Delta.Y > 0)
+                vm.PreviousBoxCommand.Execute(null);
+            else if (e.Delta.Y < 0)
+                vm.NextBoxCommand.Execute(null);
+            e.Handled = true;
+        }
     }
 }