feat(ui): D.2b empty-slot art — UiItemList.CellEmptySprite stamps cells

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/UI/UiItemList.cs

@@ -28,6 +28,23 @@ public sealed class UiItemList : UiElement
 
     public Func<uint, (uint tex, int w, int h)>? SpriteResolve { get; set; }
 
+    private uint _cellEmptySprite;
+    /// <summary>Empty-slot sprite for THIS list's cells, resolved from the dat cell template
+    /// (retail attribute 0x1000000e -> catalog 0x21000037 prototype's ItemSlot_Empty; see
+    /// <see cref="Layout.ItemListCellTemplate.ResolveEmptySprite"/>). 0 = leave the
+    /// <see cref="UiItemSlot.EmptySprite"/> default (0x060074CF). Applied to every existing
+    /// AND future cell.</summary>
+    public uint CellEmptySprite
+    {
+        get => _cellEmptySprite;
+        set
+        {
+            _cellEmptySprite = value;
+            if (value != 0)
+                foreach (var c in _cells) c.EmptySprite = value;
+        }
+    }
+
     /// <summary>The drag handler this list routes drops to (a panel controller).
     /// Null = the list accepts no drops. Retail: UIElement_ItemList::m_dragHandler.</summary>
     public IItemListDragHandler? DragHandler { get; private set; }
@@ -53,6 +70,7 @@ public sealed class UiItemList : UiElement
     public void AddItem(UiItemSlot cell)
     {
         cell.SpriteResolve ??= SpriteResolve;
+        if (_cellEmptySprite != 0) cell.EmptySprite = _cellEmptySprite;
         // The list lays cells out procedurally (grid offset + scroll clip), so cells must be
         // EXEMPT from the parent anchor pass — UiElement.DrawSelfAndChildren runs ApplyAnchor
         // on every child after OnDraw, which would otherwise reset the scroll offset to each

tests/AcDream.App.Tests/UI/UiItemListTests.cs

@@ -22,4 +22,25 @@ public class UiItemListTests
         var list = new UiItemList();
         Assert.Same(list.GetItem(0), list.Cell);
     }
+
+    [Fact]
+    public void CellEmptySprite_stamps_existing_and_future_cells()
+    {
+        var list = new UiItemList();                                 // ctor adds one default cell
+        Assert.Equal(0x060074CFu, list.GetItem(0)!.EmptySprite);     // UiItemSlot default before set
+
+        list.CellEmptySprite = 0x06004D20u;                          // a pack-slot sprite
+        Assert.Equal(0x06004D20u, list.GetItem(0)!.EmptySprite);     // existing cell re-stamped
+
+        list.AddItem(new UiItemSlot());
+        Assert.Equal(0x06004D20u, list.GetItem(1)!.EmptySprite);     // new cell stamped
+    }
+
+    [Fact]
+    public void CellEmptySprite_zero_leaves_the_UiItemSlot_default()
+    {
+        var list = new UiItemList();
+        list.CellEmptySprite = 0u;
+        Assert.Equal(0x060074CFu, list.GetItem(0)!.EmptySprite);     // unchanged default
+    }
 }