fix(docking): improve type safety, document retention, and container cleanup

src/Editor/Ghost.Editor/View/Controls/Docking/DockContainer.cs

@@ -3,6 +3,9 @@ using System.Collections.ObjectModel;
 
 namespace Ghost.Editor.View.Controls.Docking;
 
+/// <summary>
+/// Base class for containers that can hold other dock modules.
+/// </summary>
 public abstract class DockContainer : DockModule
 {
     private readonly ObservableCollection<DockModule> _children = new();
@@ -20,6 +23,11 @@ public abstract class DockContainer : DockModule
     }
 
     public virtual void AddChild(DockModule module)
+    {
+        InsertChild(_children.Count, module);
+    }
+
+    public virtual void InsertChild(int index, DockModule module)
     {
         ArgumentNullException.ThrowIfNull(module);
 
@@ -42,7 +50,8 @@ public abstract class DockContainer : DockModule
 
         module.Owner?.RemoveChild(module);
         module.Owner = this;
-        _children.Add(module);
+        module.Root = Root;
+        _children.Insert(index, module);
     }
 
     public virtual void RemoveChild(DockModule module)
@@ -52,6 +61,16 @@ public abstract class DockContainer : DockModule
         if (_children.Remove(module))
         {
             module.Owner = null;
+            module.Root = null;
+            CheckCleanup();
+        }
+    }
+
+    protected virtual void CheckCleanup()
+    {
+        if (_children.Count == 0)
+        {
+            Owner?.RemoveChild(this);
         }
     }
 
@@ -60,9 +79,18 @@ public abstract class DockContainer : DockModule
         foreach (var child in _children)
         {
             child.Owner = null;
+            child.Root = null;
         }
         _children.Clear();
     }
+
+    protected override void OnRootChanged()
+    {
+        foreach (var child in _children)
+        {
+            child.Root = Root;
+        }
+    }
     
     protected virtual void OnChildrenUpdated() { }
 }

src/Editor/Ghost.Editor/View/Controls/Docking/DockGroup.cs

@@ -4,6 +4,9 @@ using Microsoft.UI.Xaml.Data;
 
 namespace Ghost.Editor.View.Controls.Docking;
 
+/// <summary>
+/// A container that displays its children (documents) as tabs.
+/// </summary>
 [TemplatePart(Name = PART_TAB_VIEW, Type = typeof(TabView))]
 public partial class DockGroup : DockContainer
 {
@@ -27,9 +30,31 @@ public partial class DockGroup : DockContainer
         base.AddChild(module);
     }
 
+    public override void InsertChild(int index, DockModule module)
+    {
+        ArgumentNullException.ThrowIfNull(module);
+
+        if (module is not DockDocument)
+        {
+            throw new ArgumentException($"{nameof(DockGroup)} only accepts {nameof(DockDocument)} children.", nameof(module));
+        }
+
+        base.InsertChild(index, module);
+    }
+
     protected override void OnApplyTemplate()
     {
         base.OnApplyTemplate();
+
+        if (_tabView != null)
+        {
+            _tabView.TabDragStarting -= OnTabDragStarting;
+            _tabView.TabDroppedOutside -= OnTabDroppedOutside;
+            _tabView.DragOver -= OnDragOver;
+            _tabView.Drop -= OnDrop;
+            _tabView.DragLeave -= OnDragLeave;
+        }
+
         _tabView = GetTemplateChild(PART_TAB_VIEW) as TabView;
 
         if (_tabView != null)
@@ -49,7 +74,6 @@ public partial class DockGroup : DockContainer
         if (args.Tab.Tag is DockDocument doc)
         {
             args.Data.Properties.Add("DockDocument", doc);
-            doc.Detach();
         }
     }
 
@@ -65,7 +89,7 @@ public partial class DockGroup : DockContainer
     {
         if (e.DataView.Properties.ContainsKey("DockDocument"))
         {
-            e.AcceptedOperation = Windows.ApplicationModel.DataTransfer.DataPackageOperation.Move;
+            e.AcceptedOperation = global::Windows.ApplicationModel.DataTransfer.DataPackageOperation.Move;
             Root?.ShowHighlight(this, e.GetPosition(this));
         }
     }

src/Editor/Ghost.Editor/View/Controls/Docking/DockModule.cs

@@ -2,14 +2,32 @@ using Microsoft.UI.Xaml.Controls;
 
 namespace Ghost.Editor.View.Controls.Docking;
 
+/// <summary>
+/// Base class for all dockable modules in the docking system.
+/// </summary>
 public abstract class DockModule : Control
 {
     public DockContainer? Owner { get; internal set; }
     
+    private DockingLayout? _root;
+
     /// <summary>
     /// Gets or sets the root docking layout this module belongs to.
     /// </summary>
-    public DockingLayout? Root { get; internal set; }
+    public virtual DockingLayout? Root
+    {
+        get => _root;
+        internal set
+        {
+            if (_root != value)
+            {
+                _root = value;
+                OnRootChanged();
+            }
+        }
+    }
+
+    protected virtual void OnRootChanged() { }
     
     public void Detach()
     {

src/Editor/Ghost.Editor/View/Controls/Docking/DockPanel.cs

@@ -41,6 +41,27 @@ public class DockPanel : DockContainer
         UpdateLayoutStructure();
     }
 
+    protected override void CheckCleanup()
+    {
+        if (Children.Count == 0)
+        {
+            base.CheckCleanup();
+        }
+        else if (Children.Count == 1)
+        {
+            var child = Children[0];
+            var owner = Owner;
+
+            if (owner != null)
+            {
+                int index = owner.Children.IndexOf(this);
+                owner.RemoveChild(this);
+                child.Detach();
+                owner.InsertChild(index, child);
+            }
+        }
+    }
+
     private static void OnOrientationChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
     {
         ((DockPanel)d).UpdateLayoutStructure();
@@ -68,7 +89,7 @@ public class DockPanel : DockContainer
                 if (i < Children.Count - 1)
                 {
                     _grid.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Auto });
-                    var splitter = new GridSplitter { ResizeDirection = GridSplitter.GridResizeDirection.Columns, Width = SPLITTER_THICKNESS };
+                    var splitter = new CommunityToolkit.WinUI.Controls.GridSplitter { ResizeDirection = CommunityToolkit.WinUI.Controls.GridSplitter.GridResizeDirection.Columns, Width = SPLITTER_THICKNESS };
                     Grid.SetColumn(splitter, i * 2 + 1);
                     _grid.Children.Add(splitter);
                 }
@@ -86,7 +107,7 @@ public class DockPanel : DockContainer
                 if (i < Children.Count - 1)
                 {
                     _grid.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
-                    var splitter = new GridSplitter { ResizeDirection = GridSplitter.GridResizeDirection.Rows, Height = SPLITTER_THICKNESS };
+                    var splitter = new CommunityToolkit.WinUI.Controls.GridSplitter { ResizeDirection = CommunityToolkit.WinUI.Controls.GridSplitter.GridResizeDirection.Rows, Height = SPLITTER_THICKNESS };
                     Grid.SetRow(splitter, i * 2 + 1);
                     _grid.Children.Add(splitter);
                 }

src/Editor/Ghost.Editor/View/Controls/Docking/DockingLayout.cs

@@ -28,9 +28,7 @@ public class DockingLayout : Control
         set => SetValue(RootPanelProperty, value);
     }
 
-    // Used in Task 5 for drag and drop highlight
     private Canvas? _overlayCanvas;
-    // Used in Task 5 for drag and drop highlight
     private DockRegionHighlight? _highlight;
 
     public DockingLayout()
@@ -74,11 +72,6 @@ public class DockingLayout : Control
     /// <param name="targetGroup">The target group to add the document to. If null, a suitable group will be found or created.</param>
     public void AddDocument(DockDocument document, DockTarget target, DockGroup? targetGroup = null)
     {
-        if (target != DockTarget.Center)
-        {
-            throw new NotImplementedException("Target docking will be implemented in Task 5");
-        }
-
         if (targetGroup != null && targetGroup.Root != this)
         {
             throw new ArgumentException("targetGroup does not belong to this DockingLayout");
@@ -100,7 +93,77 @@ public class DockingLayout : Control
             }
         }
 
-        targetGroup.AddChild(document);
+        if (target == DockTarget.Center)
+        {
+            targetGroup.AddChild(document);
+        }
+        else
+        {
+            SplitGroup(targetGroup, document, target);
+        }
+    }
+
+    private void SplitGroup(DockGroup targetGroup, DockDocument doc, DockTarget target)
+    {
+        var parentPanel = targetGroup.Owner as DockPanel;
+        var newGroup = new DockGroup();
+        newGroup.AddChild(doc);
+
+        var orientation = (target == DockTarget.Left || target == DockTarget.Right) ? Orientation.Horizontal : Orientation.Vertical;
+
+        if (parentPanel == null)
+        {
+            // targetGroup is the root
+            var newPanel = new DockPanel { Orientation = orientation };
+            RootPanel = newPanel;
+
+            if (target == DockTarget.Left || target == DockTarget.Top)
+            {
+                newPanel.AddChild(newGroup);
+                newPanel.AddChild(targetGroup);
+            }
+            else
+            {
+                newPanel.AddChild(targetGroup);
+                newPanel.AddChild(newGroup);
+            }
+        }
+        else
+        {
+            int index = parentPanel.Children.IndexOf(targetGroup);
+
+            if (parentPanel.Orientation == orientation)
+            {
+                // Same orientation, just insert
+                if (target == DockTarget.Left || target == DockTarget.Top)
+                {
+                    parentPanel.InsertChild(index, newGroup);
+                }
+                else
+                {
+                    parentPanel.InsertChild(index + 1, newGroup);
+                }
+            }
+            else
+            {
+                // Different orientation, need a new sub-panel
+                targetGroup.Detach();
+                var newPanel = new DockPanel { Orientation = orientation };
+
+                if (target == DockTarget.Left || target == DockTarget.Top)
+                {
+                    newPanel.AddChild(newGroup);
+                    newPanel.AddChild(targetGroup);
+                }
+                else
+                {
+                    newPanel.AddChild(targetGroup);
+                    newPanel.AddChild(newGroup);
+                }
+
+                parentPanel.InsertChild(index, newPanel);
+            }
+        }
     }
 
     private static DockGroup? FindFirstDockGroup(DockContainer container)
@@ -125,7 +188,7 @@ public class DockingLayout : Control
         return null;
     }
 
-    internal void ShowHighlight(DockGroup targetGroup, Windows.Foundation.Point position)
+    internal void ShowHighlight(DockGroup targetGroup, global::Windows.Foundation.Point position)
     {
         if (_highlight == null || _overlayCanvas == null) return;
 
@@ -147,10 +210,10 @@ public class DockingLayout : Control
         }
 
         var transform = targetGroup.TransformToVisual(_overlayCanvas);
-        var point = transform.TransformPoint(new Windows.Foundation.Point(x, y));
+        var point = transform.TransformPoint(new global::Windows.Foundation.Point(x, y));
 
-        Canvas.SetLeft(_highlight, point.X);
+        Microsoft.UI.Xaml.Controls.Canvas.SetLeft(_highlight, point.X);
-        Canvas.SetTop(_highlight, point.Y);
+        Microsoft.UI.Xaml.Controls.Canvas.SetTop(_highlight, point.Y);
         _highlight.Width = width;
         _highlight.Height = height;
     }
@@ -160,45 +223,24 @@ public class DockingLayout : Control
         if (_highlight != null) _highlight.Visibility = Visibility.Collapsed;
     }
 
-    internal void HandleDrop(DockDocument doc, DockGroup targetGroup, Windows.Foundation.Point position)
+    internal void HandleDrop(DockDocument doc, DockGroup targetGroup, global::Windows.Foundation.Point position)
     {
         HideHighlight();
         var target = CalculateDockTarget(targetGroup, position);
 
+        doc.Detach();
+
         if (target == DockTarget.Center)
         {
             targetGroup.AddChild(doc);
         }
         else
         {
-            // Split logic: create new DockPanel, move targetGroup and doc into it
+            SplitGroup(targetGroup, doc, target);
-            var parentPanel = targetGroup.Owner as DockPanel;
-            if (parentPanel != null)
-            {
-                int index = parentPanel.Children.IndexOf(targetGroup);
-                targetGroup.Detach();
-
-                var newPanel = new DockPanel { Orientation = (target == DockTarget.Left || target == DockTarget.Right) ? Orientation.Horizontal : Orientation.Vertical };
-                var newGroup = new DockGroup();
-                newGroup.AddChild(doc);
-
-                if (target == DockTarget.Left || target == DockTarget.Top)
-                {
-                    newPanel.AddChild(newGroup);
-                    newPanel.AddChild(targetGroup);
-                }
-                else
-                {
-                    newPanel.AddChild(targetGroup);
-                    newPanel.AddChild(newGroup);
-                }
-
-                parentPanel.Children.Insert(index, newPanel);
-            }
         }
     }
 
-    private DockTarget CalculateDockTarget(DockGroup group, Windows.Foundation.Point position)
+    private DockTarget CalculateDockTarget(DockGroup group, global::Windows.Foundation.Point position)
     {
         double w = group.ActualWidth;
         double h = group.ActualHeight;
@@ -214,6 +256,7 @@ public class DockingLayout : Control
 
     internal void CreateFloatingWindow(DockDocument doc)
     {
+        doc.Detach();
         // To be implemented in Task 6
     }
 }