feat: drag-drop sprites from sprite list to composition grid

- Manual drag with floating popup showing sprite image (not generic file icon)
- Hit-test drop on composition cells to assign sprite to slot
- FrameGroup.GetFlatIndex() and SetSpriteId() for slot-level writes
- SpriteViewModel.SlotIndex tracks position in SpriteIndex array
- Pixel-perfect drag preview with BitmapInterpolationMode.None

Author: BrewertonSantos

src/App/MainWindow.axaml

@@ -578,7 +578,7 @@
                                 Height="{Binding CompositionPreviewHeight}"
                                 HorizontalAlignment="Center" VerticalAlignment="Center">
                             <Border Background="{Binding CompositionGridBrush}">
-                              <ItemsControl ItemsSource="{Binding CompositionSprites}">
+                              <ItemsControl x:Name="CompositionGrid" ItemsSource="{Binding CompositionSprites}">
                                 <ItemsControl.ItemsPanel>
                                   <ItemsPanelTemplate><WrapPanel/></ItemsPanelTemplate>
                                 </ItemsControl.ItemsPanel>
@@ -1420,7 +1420,10 @@
                   <DataTemplate x:DataType="vm:SpriteViewModel">
                     <Border Width="38" Height="46" Margin="1" Background="#11111b"
                             CornerRadius="3" BorderBrush="#313244" BorderThickness="1"
-                            ToolTip.Tip="{Binding SpriteId}">
+                            ToolTip.Tip="{Binding SpriteId}"
+                            PointerPressed="OnRightSpritePointerPressed"
+                            PointerMoved="OnRightSpritePointerMoved"
+                            PointerReleased="OnRightSpritePointerReleased">
                       <Border.ContextMenu>
                         <ContextMenu>
                           <MenuItem Header="Replace" Command="{Binding $parent[ListBox].((vm:MainWindowViewModel)DataContext).ReplaceSpriteCommand}"/>

src/App/MainWindow.axaml.cs

@@ -21,6 +21,7 @@ public MainWindow()
     {
         InitializeComponent();
         Closing += OnWindowClosing;
+
         Loaded += async (_, _) =>
         {
             if (DataContext is MainWindowViewModel vm)
@@ -294,6 +295,105 @@ private void OnCompositionSpriteDoubleTapped(object? sender, TappedEventArgs e)
         }
     }
 
+    // ── Sprite drag-drop: drag from sprite list → drop on composition cell ──
+
+    private SpriteViewModel? _dragSprite;
+    private Popup? _dragAdorner;
+    private bool _spriteDragActive;
+
+    private void OnRightSpritePointerPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (sender is not Border border || border.DataContext is not SpriteViewModel svm)
+            return;
+
+        var props = e.GetCurrentPoint(border).Properties;
+        if (!props.IsLeftButtonPressed) return;
+
+        _dragSprite = svm;
+        _spriteDragActive = false;
+        e.Pointer.Capture(border);
+        e.Handled = true;
+    }
+
+    private void OnRightSpritePointerMoved(object? sender, PointerEventArgs e)
+    {
+        if (_dragSprite == null) return;
+
+        if (!_spriteDragActive)
+        {
+            _spriteDragActive = true;
+            // Create floating adorner showing the sprite image
+            var img = new Image
+            {
+                Source = _dragSprite.Bitmap,
+                Width = 32,
+                Height = 32,
+                Stretch = Stretch.None
+            };
+            RenderOptions.SetBitmapInterpolationMode(img, Avalonia.Media.Imaging.BitmapInterpolationMode.None);
+            _dragAdorner = new Popup
+            {
+                IsLightDismissEnabled = false,
+                Placement = PlacementMode.Pointer,
+                PlacementTarget = this,
+                Child = new Border
+                {
+                    Background = Brushes.Transparent,
+                    Opacity = 0.85,
+                    IsHitTestVisible = false,
+                    Child = img
+                }
+            };
+            ((Panel)this.Content!).Children.Add(_dragAdorner);
+            _dragAdorner.Open();
+        }
+
+        // Reposition by re-opening at pointer
+        if (_dragAdorner != null)
+        {
+            _dragAdorner.Close();
+            _dragAdorner.Open();
+        }
+    }
+
+    private void OnRightSpritePointerReleased(object? sender, PointerReleasedEventArgs e)
+    {
+        if (_dragSprite == null) return;
+
+        var draggedSprite = _dragSprite;
+        _dragSprite = null;
+        _spriteDragActive = false;
+
+        // Close and remove adorner
+        if (_dragAdorner != null)
+        {
+            _dragAdorner.Close();
+            if (this.Content is Panel panel)
+                panel.Children.Remove(_dragAdorner);
+            _dragAdorner = null;
+        }
+
+        e.Pointer.Capture(null);
+
+        // Hit-test: find the composition cell under the pointer
+        var pos = e.GetPosition(this);
+        var hit = this.InputHitTest(pos);
+        var visual = hit as Control;
+        SpriteViewModel? targetSlot = null;
+        while (visual != null)
+        {
+            if (visual.DataContext is SpriteViewModel svm && svm.SlotIndex >= 0)
+            {
+                targetSlot = svm;
+                break;
+            }
+            visual = visual.Parent as Control;
+        }
+
+        if (targetSlot != null && DataContext is MainWindowViewModel vm)
+            vm.AssignSpriteToSlot(targetSlot, draggedSprite.SpriteId);
+    }
+
     private void OnClientItemDoubleTapped(object? sender, TappedEventArgs e)
     {
         if (DataContext is MainWindowViewModel vm)

src/App/ViewModels/MainWindowViewModel.cs

@@ -3355,8 +3355,9 @@ private void BuildCompositionGrid()
                     {
                         int px = pxStart + patX;
                         int py = pyStart + patY;
-                        uint spriteId = fg.GetSpriteId(fg.Width - 1 - col, fg.Height - 1 - row, layer, px, py, pz, frame);
-                        var svm = new SpriteViewModel { SpriteId = spriteId };
+                        int flatIdx = fg.GetFlatIndex(fg.Width - 1 - col, fg.Height - 1 - row, layer, px, py, pz, frame);
+                        uint spriteId = flatIdx >= 0 ? fg.SpriteIndex[flatIdx] : 0;
+                        var svm = new SpriteViewModel { SpriteId = spriteId, SlotIndex = flatIdx };
                         svm.Bitmap = LoadSpriteBitmap(spriteId);
                         CompositionSprites.Add(svm);
                     }
@@ -3380,6 +3381,25 @@ private void NavigateRightSpriteToId(uint spriteId)
         SelectedRightSprite = RightSprites.FirstOrDefault(s => s.SpriteId == spriteId);
     }
 
+    /// <summary>Assign a sprite to a composition slot (used by drag-drop from sprite list).</summary>
+    public void AssignSpriteToSlot(SpriteViewModel targetSlot, uint newSpriteId)
+    {
+        if (targetSlot.SlotIndex < 0) return;
+        var thing = _currentCompositionThing;
+        if (thing == null || thing.FrameGroups.Length == 0) return;
+
+        int fgIdx = Math.Clamp(CompositionFrameGroupIndex, 0, Math.Max(0, thing.FrameGroups.Length - 1));
+        var fg = thing.FrameGroups[fgIdx];
+
+        fg.SetSpriteId(targetSlot.SlotIndex, newSpriteId);
+
+        // Update the cell in-place
+        targetSlot.SpriteId = newSpriteId;
+        targetSlot.Bitmap = LoadSpriteBitmap(newSpriteId);
+
+        StatusText = $"Set slot {targetSlot.SlotIndex} → sprite {newSpriteId}";
+    }
+
     // ══════════════════════════════════════════════════════════════════════
     // ── Reset thing to original state ──
     // ══════════════════════════════════════════════════════════════════════

src/App/ViewModels/SpriteViewModel.cs

@@ -11,4 +11,7 @@ public partial class SpriteViewModel : ObservableObject
 {
     [ObservableProperty] private uint _spriteId;
     [ObservableProperty] private WriteableBitmap? _bitmap;
+
+    /// <summary>Flat index into FrameGroup.SpriteIndex[] (set only for composition cells).</summary>
+    public int SlotIndex { get; set; } = -1;
 }

src/OTB/DatThingType.cs

@@ -213,6 +213,23 @@ public uint GetSpriteId(int w, int h, int layer, int patternX, int patternY, int
         return index < SpriteIndex.Length ? SpriteIndex[index] : 0;
     }
 
+    /// <summary>Compute the flat index for a given position (same formula as GetSpriteId).</summary>
+    public int GetFlatIndex(int w, int h, int layer, int patternX, int patternY, int patternZ, int frame)
+    {
+        if (Width == 0 || Height == 0 || Layers == 0 || PatternX == 0 || PatternY == 0 || PatternZ == 0 || Frames == 0)
+            return -1;
+        int index = ((((((frame % Frames) * PatternZ + patternZ) * PatternY + patternY) * PatternX + patternX)
+                     * Layers + layer) * Height + h) * Width + w;
+        return index < SpriteIndex.Length ? index : -1;
+    }
+
+    /// <summary>Set sprite ID at a flat index position.</summary>
+    public void SetSpriteId(int flatIndex, uint spriteId)
+    {
+        if (flatIndex >= 0 && flatIndex < SpriteIndex.Length)
+            SpriteIndex[flatIndex] = spriteId;
+    }
+
     /// <summary>Creates a deep copy of this frame group.</summary>
     public FrameGroup Clone()
     {