Fix clearing nested selection not raising SelectionChanged.

src/Avalonia.Controls/SelectionModel.cs

@@ -324,6 +324,7 @@ namespace Avalonia.Controls
         public void Dispose()
         {
             ClearSelection(resetAnchor: false);
+            _rootNode.Cleanup();
             _rootNode.Dispose();
             _selectedIndicesCached = null;
             _selectedItemsCached = null;
@@ -764,6 +765,7 @@ namespace Avalonia.Controls
             }
 
             OnSelectionChanged(e);
+            _rootNode.Cleanup();
         }
 
         internal class SelectedItemInfo : ISelectedItemInfo

src/Avalonia.Controls/SelectionNode.cs

@@ -342,6 +342,19 @@ namespace Avalonia.Controls
             }
         }
 
+        public void Cleanup()
+        {
+            foreach (var child in _childrenNodes)
+            {
+                child?.Cleanup();
+
+                if (child?.SelectedCount == 0)
+                {
+                    child.Dispose();
+                }
+            }
+        }
+
         public bool Select(int index, bool select)
         {
             return Select(index, select, raiseOnSelectionChanged: true);
@@ -453,16 +466,6 @@ namespace Avalonia.Controls
 
             SelectedCount = 0;
             AnchorIndex = -1;
-
-            // This will throw away all the children SelectionNodes
-            // causing them to be unhooked from their data source. This
-            // essentially cleans up the tree.
-            foreach (var child in _childrenNodes)
-            {
-                child?.Dispose();
-            }
-
-            _childrenNodes.Clear();
         }
 
         private bool Select(int index, bool select, bool raiseOnSelectionChanged)