feat(ui): D.2b-B — sub-window mount (inheritor attaches base subtree)

LayoutImporter.Resolve now captures the resolved base element's children
and, for a pure-container leaf (no own children + no own media) inheriting
from a base WITH content, attaches that subtree. This pulls the nested
gmInventoryUI panels' content (paperdoll/backpack/3D-items) in through the
existing BaseElement+BaseLayoutId path. ShouldMountBaseChildren is a pure,
unit-tested predicate; it's inert for media-bearing inheritors and childless
style prototypes, so vitals/chat/toolbar are unaffected (existing importer
tests still green).

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

src/AcDream.App/UI/Layout/LayoutImporter.cs

@@ -191,6 +191,14 @@ public static class LayoutImporter
 
     // ── Inheritance resolution ────────────────────────────────────────────────
 
+    /// <summary>True when a pure-container leaf should inherit its base's subtree (the
+    /// gmInventoryUI sub-window mount): the derived element has no own children, no own
+    /// state media, and the base resolved to ≥1 child. Inert for media-bearing inheritors
+    /// (close button / title), elements with their own children, and childless style
+    /// prototypes (vitals/chat/toolbar text — the base prototype has no children).</summary>
+    internal static bool ShouldMountBaseChildren(int derivedChildCount, int derivedMediaCount, int baseChildCount)
+        => derivedChildCount == 0 && derivedMediaCount == 0 && baseChildCount > 0;
+
     /// <summary>
     /// Converts an <see cref="ElementDesc"/> to a resolved <see cref="ElementInfo"/>:
     /// reads own fields + media, applies the BaseElement / BaseLayoutId chain
@@ -204,6 +212,7 @@ public static class LayoutImporter
         // Read this element's own fields + media (no inheritance, no children yet).
         var self = ToInfo(d);
         var result = self;
+        List<ElementInfo>? baseChildren = null;
 
         // Apply BaseElement / BaseLayoutId inheritance if present.
         if (d.BaseElement != 0 && d.BaseLayoutId != 0
@@ -215,9 +224,9 @@ public static class LayoutImporter
             {
                 // Recurse the base chain (already guarded by the HashSet add above).
                 var baseInfo = Resolve(dats, baseDesc, baseChain);
-                // Derived fields override the base; result.Children is still empty here
-                // — children are attached below from the DERIVED element's own tree.
+                // Derived fields override the base; children are attached below.
                 result = ElementReader.Merge(baseInfo, self);
+                baseChildren = baseInfo.Children;   // capture for the sub-window mount
             }
         }
 
@@ -226,6 +235,15 @@ public static class LayoutImporter
         foreach (var kv in d.Children)
             result.Children.Add(Resolve(dats, kv.Value, new HashSet<(uint, uint)>()));
 
+        // Sub-window mount: a pure-container leaf (no own children, no own media) that inherits
+        // from a base WITH content attaches the base's subtree. Targets the gmInventoryUI panels
+        // (0x100001CD/CE/CF — Type-0, media-less, childless, BaseLayoutId → a gm*UI window);
+        // inert for media-bearing inheritors (close button/title) and childless style prototypes
+        // (vitals/chat/toolbar). See ShouldMountBaseChildren + the B-Grid spec.
+        if (baseChildren is not null
+            && ShouldMountBaseChildren(d.Children.Count, self.StateMedia.Count, baseChildren.Count))
+            result.Children.AddRange(baseChildren);
+
         return result;
     }
 

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

@@ -0,0 +1,22 @@
+using AcDream.App.UI.Layout;
+
+namespace AcDream.App.Tests.UI;
+
+public class LayoutImporterMountTests
+{
+    [Fact]
+    public void Mounts_ChildlessMediaLessInheritor_WithContentfulBase()
+        => Assert.True(LayoutImporter.ShouldMountBaseChildren(derivedChildCount: 0, derivedMediaCount: 0, baseChildCount: 5));
+
+    [Fact]
+    public void DoesNotMount_WhenDerivedHasOwnMedia()        // close button / title
+        => Assert.False(LayoutImporter.ShouldMountBaseChildren(0, 1, 5));
+
+    [Fact]
+    public void DoesNotMount_WhenDerivedHasOwnChildren()
+        => Assert.False(LayoutImporter.ShouldMountBaseChildren(2, 0, 5));
+
+    [Fact]
+    public void DoesNotMount_WhenBaseIsChildless()           // vitals/chat/toolbar style prototypes
+        => Assert.False(LayoutImporter.ShouldMountBaseChildren(0, 0, 0));
+}