test(dock): add model-level mutation and cleanup tests

src/Test/Ghost.UnitTest/DockingMutationTest.cs

@@ -0,0 +1,120 @@
+using Ghost.Editor.Core.Controls.Internal.Docking;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace Ghost.UnitTest;
+
+[TestClass]
+public class DockingMutationTest
+{
+    [TestMethod]
+    public void TestCenterDrop_MovesItem()
+    {
+        var root = new DockGroupNode();
+        var panel1 = new DockPanelNode();
+        var panel2 = new DockPanelNode();
+        var item = new object();
+        
+        panel1.Items.Add(item);
+        root.AddChild(panel1);
+        root.AddChild(panel2);
+
+        // Simulate Center Drop: panel1 -> panel2
+        panel1.Items.Remove(item);
+        panel2.Items.Add(item);
+
+        Assert.AreEqual(0, panel1.Items.Count);
+        Assert.AreEqual(1, panel2.Items.Count);
+        Assert.AreEqual(item, panel2.Items[0]);
+    }
+
+    [TestMethod]
+    public void TestSplitDrop_CreatesNewStructure()
+    {
+        var root = new DockGroupNode { Orientation = Microsoft.UI.Xaml.Controls.Orientation.Horizontal };
+        var panel1 = new DockPanelNode();
+        var item = new object();
+        
+        panel1.Items.Add(item);
+        root.AddChild(panel1);
+
+        // Simulate Vertical Split Drop (Bottom) on panel1
+        // 1. Remove from source
+        panel1.Items.Remove(item);
+        
+        // 2. Different orientation (Vertical), replace panel1 with new group
+        root.RemoveChild(panel1);
+        var newGroup = new DockGroupNode { Orientation = Microsoft.UI.Xaml.Controls.Orientation.Vertical };
+        var newNode = new DockPanelNode();
+        newNode.Items.Add(item);
+        
+        newGroup.AddChild(panel1); // Original
+        newGroup.AddChild(newNode); // New split
+        root.AddChild(newGroup);
+
+        Assert.AreEqual(1, root.Children.Count);
+        Assert.AreEqual(newGroup, root.Children[0]);
+        Assert.AreEqual(2, newGroup.Children.Count);
+        Assert.AreEqual(panel1, newGroup.Children[0]);
+        Assert.AreEqual(newNode, newGroup.Children[1]);
+        Assert.AreEqual(item, newNode.Items[0]);
+    }
+
+    [TestMethod]
+    public void TestCleanup_Cascades()
+    {
+        var root = new DockGroupNode();
+        var group1 = new DockGroupNode();
+        var panel1 = new DockPanelNode();
+        
+        group1.AddChild(panel1);
+        root.AddChild(group1);
+
+        // panel1 becomes empty
+        panel1.Items.Clear();
+        
+        // Simulate CleanupEmptyNodes(panel1)
+        // 1. panel1 is empty, remove from group1
+        group1.RemoveChild(panel1);
+        
+        // 2. group1 is now empty, remove from root
+        if (group1.Children.Count == 0)
+        {
+            root.RemoveChild(group1);
+        }
+
+        Assert.AreEqual(0, root.Children.Count);
+        Assert.IsNull(group1.Parent);
+        Assert.IsNull(panel1.Parent);
+    }
+
+    [TestMethod]
+    public void TestCleanup_CollapsesRedundantGroup()
+    {
+        var root = new DockGroupNode();
+        var group1 = new DockGroupNode();
+        var panel1 = new DockPanelNode();
+        var panel2 = new DockPanelNode();
+        
+        group1.AddChild(panel1);
+        group1.AddChild(panel2);
+        root.AddChild(group1);
+
+        // panel2 is removed
+        group1.RemoveChild(panel2);
+        
+        // group1 now has only 1 child (panel1), should collapse
+        if (group1.Children.Count == 1)
+        {
+            var onlyChild = group1.Children[0];
+            var parent = group1.Parent;
+            int index = parent.Children.IndexOf(group1);
+            
+            parent.RemoveChild(group1);
+            parent.InsertChild(index, onlyChild);
+        }
+
+        Assert.AreEqual(1, root.Children.Count);
+        Assert.AreEqual(panel1, root.Children[0]);
+        Assert.AreEqual(root, panel1.Parent);
+    }
+}