fix(dock): robust selection sync, internal parent setter, and XML docs

src/Editor/Ghost.Editor.Core/Controls/Internal/Docking/DockGroupNode.cs

@@ -4,19 +4,38 @@ using Microsoft.UI.Xaml.Controls;
 
 namespace Ghost.Editor.Core.Controls.Internal.Docking;
 
+/// <summary>
+/// A docking node that contains multiple children and arranges them in a specific orientation.
+/// </summary>
 public partial class DockGroupNode : DockNode
 {
+    /// <summary>
+    /// Gets or sets the layout orientation of the children.
+    /// </summary>
     [ObservableProperty]
     public partial Orientation Orientation { get; set; }
 
+    /// <summary>
+    /// Gets the collection of child nodes.
+    /// </summary>
+    public ObservableCollection<DockNode> Children { get; } = new();
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="DockGroupNode"/> class.
+    /// </summary>
     public DockGroupNode()
     {
         Orientation = Orientation.Horizontal;
     }
 
-    public ObservableCollection<DockNode> Children { get; } = new();
+    /// <summary>
-
+    /// Adds a child node to this group, enforcing tree invariants.
+    /// </summary>
+    /// <param name="node">The node to add.</param>
+    /// <exception cref="ArgumentNullException">Thrown if node is null.</exception>
+    /// <exception cref="InvalidOperationException">Thrown if adding the node would create a cycle or if adding self.</exception>
     public void AddChild(DockNode node)
+
     {
         ArgumentNullException.ThrowIfNull(node);
 
@@ -50,6 +69,10 @@ public partial class DockGroupNode : DockNode
         Children.Add(node);
     }
 
+    /// <summary>
+    /// Removes a child node from this group.
+    /// </summary>
+    /// <param name="node">The node to remove.</param>
     public void RemoveChild(DockNode node)
     {
         if (Children.Remove(node))

src/Editor/Ghost.Editor.Core/Controls/Internal/Docking/DockNode.cs

@@ -2,8 +2,14 @@ using CommunityToolkit.Mvvm.ComponentModel;
 
 namespace Ghost.Editor.Core.Controls.Internal.Docking;
 
+/// <summary>
+/// Base class for all nodes in the docking layout tree.
+/// </summary>
 public abstract partial class DockNode : ObservableObject
 {
+    /// <summary>
+    /// Gets the parent group of this node.
+    /// </summary>
     [ObservableProperty]
-    public partial DockGroupNode? Parent { get; set; }
+    public partial DockGroupNode? Parent { get; internal set; }
 }

src/Editor/Ghost.Editor.Core/Controls/Internal/Docking/DockPanelNode.cs

@@ -3,16 +3,31 @@ using CommunityToolkit.Mvvm.ComponentModel;
 
 namespace Ghost.Editor.Core.Controls.Internal.Docking;
 
+/// <summary>
+/// A docking node that contains a collection of items (tabs) and manages selection.
+/// </summary>
 public partial class DockPanelNode : DockNode
 {
+    /// <summary>
+    /// Gets the collection of items (tabs) in this panel.
+    /// </summary>
     public ObservableCollection<object> Items { get; } = new();
 
+    /// <summary>
+    /// Gets or sets the index of the currently selected item.
+    /// </summary>
     [ObservableProperty]
     public partial int SelectedIndex { get; set; }
 
+    /// <summary>
+    /// Gets or sets the currently selected item.
+    /// </summary>
     [ObservableProperty]
     public partial object? SelectedItem { get; set; }
 
+    /// <summary>
+    /// Initializes a new instance of the <see cref="DockPanelNode"/> class.
+    /// </summary>
     public DockPanelNode()
     {
         SelectedIndex = -1;
@@ -21,18 +36,38 @@ public partial class DockPanelNode : DockNode
 
     private void OnItemsCollectionChanged(object? sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
     {
+        // Reconcile selection on every collection change
         if (Items.Count == 0)
         {
             SelectedIndex = -1;
             SelectedItem = null;
+            return;
         }
-        else if (SelectedIndex >= Items.Count)
+
+        if (SelectedItem != null)
         {
-            SelectedIndex = Items.Count - 1;
+            int index = Items.IndexOf(SelectedItem);
+            if (index != -1)
+            {
+                // Item still exists, update index if it changed
+                if (SelectedIndex != index)
+                {
+                    SelectedIndex = index;
+                }
+                return;
+            }
         }
-        else if (SelectedIndex == -1 && Items.Count > 0)
+
+        // SelectedItem is null or no longer in collection
+        if (SelectedIndex >= 0 && SelectedIndex < Items.Count)
         {
-            SelectedIndex = 0;
+            // Keep current index if valid, update item
+            SelectedItem = Items[SelectedIndex];
+        }
+        else
+        {
+            // Fallback to first item or -1
+            SelectedIndex = Items.Count > 0 ? 0 : -1;
         }
     }
 
@@ -40,11 +75,18 @@ public partial class DockPanelNode : DockNode
     {
         if (value >= 0 && value < Items.Count)
         {
-            SelectedItem = Items[value];
+            object newItem = Items[value];
+            if (SelectedItem != newItem)
+            {
+                SelectedItem = newItem;
+            }
         }
         else if (value == -1)
         {
-            SelectedItem = null;
+            if (SelectedItem != null)
+            {
+                SelectedItem = null;
+            }
         }
         else
         {
@@ -57,14 +99,26 @@ public partial class DockPanelNode : DockNode
     {
         if (value == null)
         {
-            SelectedIndex = -1;
+            if (SelectedIndex != -1)
+            {
+                SelectedIndex = -1;
+            }
         }
         else
         {
             int index = Items.IndexOf(value);
             if (index != -1)
             {
-                SelectedIndex = index;
+                if (SelectedIndex != index)
+                {
+                    SelectedIndex = index;
+                }
+            }
+            else
+            {
+                // Item not in collection - reject selection
+                SelectedItem = null;
+                SelectedIndex = -1;
             }
         }
     }

src/Test/Ghost.UnitTest/DockingModelTest.cs

@@ -16,7 +16,7 @@ public class DockingModelTest
         group.AddChild(child);
 
         Assert.AreEqual(group, child.Parent);
-        Assert.IsTrue(group.Children.Contains(child));
+        CollectionAssert.Contains(group.Children, child);
     }
 
     [TestMethod]
@@ -30,8 +30,8 @@ public class DockingModelTest
         group2.AddChild(child);
 
         Assert.AreEqual(group2, child.Parent);
-        Assert.IsFalse(group1.Children.Contains(child));
+        CollectionAssert.DoesNotContain(group1.Children, child);
-        Assert.IsTrue(group2.Children.Contains(child));
+        CollectionAssert.Contains(group2.Children, child);
     }
 
     [TestMethod]
@@ -63,7 +63,7 @@ public class DockingModelTest
         group.RemoveChild(child);
 
         Assert.IsNull(child.Parent);
-        Assert.IsFalse(group.Children.Contains(child));
+        CollectionAssert.DoesNotContain(group.Children, child);
     }
 
     [TestMethod]
@@ -120,4 +120,59 @@ public class DockingModelTest
         Assert.AreEqual(-1, panel.SelectedIndex);
         Assert.IsNull(panel.SelectedItem);
     }
+
+    [TestMethod]
+    public void TestPanel_RemoveMiddleItem_MaintainsSelection()
+    {
+        var panel = new DockPanelNode();
+        var item1 = new object();
+        var item2 = new object();
+        var item3 = new object();
+
+        panel.Items.Add(item1);
+        panel.Items.Add(item2);
+        panel.Items.Add(item3);
+
+        panel.SelectedItem = item2;
+        Assert.AreEqual(1, panel.SelectedIndex);
+
+        // Remove item1 (before selection)
+        panel.Items.Remove(item1);
+        Assert.AreEqual(item2, panel.SelectedItem);
+        Assert.AreEqual(0, panel.SelectedIndex);
+    }
+
+    [TestMethod]
+    public void TestPanel_RemoveSelectedItem_UpdatesSelection()
+    {
+        var panel = new DockPanelNode();
+        var item1 = new object();
+        var item2 = new object();
+
+        panel.Items.Add(item1);
+        panel.Items.Add(item2);
+
+        panel.SelectedItem = item1;
+        panel.Items.Remove(item1);
+
+        // Should fallback to next available item at same index
+        Assert.AreEqual(item2, panel.SelectedItem);
+        Assert.AreEqual(0, panel.SelectedIndex);
+    }
+
+    [TestMethod]
+    public void TestPanel_SetInvalidSelectedItem_ResetsSelection()
+    {
+        var panel = new DockPanelNode();
+        var item1 = new object();
+        var item2 = new object();
+
+        panel.Items.Add(item1);
+        panel.SelectedItem = item1;
+
+        panel.SelectedItem = item2; // Not in collection
+
+        Assert.IsNull(panel.SelectedItem);
+        Assert.AreEqual(-1, panel.SelectedIndex);
+    }
 }