feat(D.2b): UiItemSlot open-container triangle + selected-item square overlays

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

src/AcDream.App/UI/UiItemSlot.cs

@@ -38,6 +38,23 @@ public sealed class UiItemSlot : UiElement
     /// state id 0x10000040).</summary>
     public uint DragRejectSprite { get; set; } = 0x060011F8u;
 
+    /// <summary>True when this cell is the OPEN container (its contents fill the grid). Draws the
+    /// open-container triangle. Port of UIElement_ItemList::UpdateOpenContainerIndicator
+    /// (0x004e3070) → SetOpenContainerState (0x004e1200): shown when item.itemID == openContainerId.</summary>
+    public bool IsOpenContainer { get; set; }
+    /// <summary>Open-container triangle sprite (element 0x10000450 on the container prototype
+    /// 0x1000033F). Configurable; guard id != 0 before resolving.</summary>
+    public uint OpenContainerSprite { get; set; } = 0x06005D9Cu;
+
+    /// <summary>True when this cell is the SELECTED item. Draws the green/yellow selection square.
+    /// Port of UIElement_ItemList::ItemList_SetSelectedItem (0x004e2fe0) → SetSelectedState
+    /// (0x004e1240): shown when item.itemID == selectedItemId. Uniform across item + container cells.</summary>
+    public bool Selected { get; set; }
+    /// <summary>Selected-item square sprite (element 0x10000342 on the 32×32 item prototype
+    /// 0x10000341; pixel-confirmed green/yellow frame). Drawn as a procedural overlay so it renders
+    /// on the 36×36 container cell too (whose prototype lacks the square child).</summary>
+    public uint SelectedSprite { get; set; } = 0x06004D21u;
+
     /// <summary>Accept/reject overlay state while a drag hovers this cell.</summary>
     public enum DragAcceptState { None, Accept, Reject }
     private DragAcceptState _dragAccept = DragAcceptState.None;
@@ -220,6 +237,23 @@ public sealed class UiItemSlot : UiElement
             }
         }
 
+        // Open-container triangle (retail SetOpenContainerState 0x004e1200) + selected-item square
+        // (retail SetSelectedState 0x004e1240). Drawn procedurally like the digit/drag overlays;
+        // guard id != 0 BEFORE resolving (feedback_ui_resolve_zero_magenta). Triangle under the
+        // square; both under the transient drag overlay below.
+        if (IsOpenContainer && SpriteResolve is not null && OpenContainerSprite != 0)
+        {
+            var (tex, _, _) = SpriteResolve(OpenContainerSprite);
+            if (tex != 0)
+                ctx.DrawSprite(tex, 0f, 0f, Width, Height, 0f, 0f, 1f, 1f, Vector4.One);
+        }
+        if (Selected && SpriteResolve is not null && SelectedSprite != 0)
+        {
+            var (tex, _, _) = SpriteResolve(SelectedSprite);
+            if (tex != 0)
+                ctx.DrawSprite(tex, 0f, 0f, Width, Height, 0f, 0f, 1f, 1f, Vector4.One);
+        }
+
         // Drag-rollover accept/reject frame (retail SetDragAcceptState 0x10000041/40).
         // Guard id != 0 BEFORE resolving — resolve(0) returns the 1×1 magenta placeholder
         // with a non-zero GL handle (feedback_ui_resolve_zero_magenta).

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

@@ -141,4 +141,20 @@ public class UiItemSlotTests
         s.SetShortcutNum(0, peace: true);
         Assert.Null(s.ActiveDigitArray());
     }
+
+    // ── Container-switching indicator overlays (decomp SetOpenContainerState 0x004e1200 /
+    //    SetSelectedState 0x004e1240) ───────────────────────────────────────────────────────
+    [Fact]
+    public void Selected_defaultsFalse() => Assert.False(new UiItemSlot().Selected);
+
+    [Fact]
+    public void IsOpenContainer_defaultsFalse() => Assert.False(new UiItemSlot().IsOpenContainer);
+
+    [Fact]
+    public void SelectedSprite_default_isGreenYellowSquare()
+        => Assert.Equal(0x06004D21u, new UiItemSlot().SelectedSprite);
+
+    [Fact]
+    public void OpenContainerSprite_default_isTriangle()
+        => Assert.Equal(0x06005D9Cu, new UiItemSlot().OpenContainerSprite);
 }