Correctly recycle cells with focus.

Previously, a cell with focus wasn't being recycled when the parent row was removed as `TreeDataGridPresenterBase.RecycleElement` was being called on it, which has logic to keep the focused element around. This makes no sense when the containing row has been removed, instead `RecycleElementOnItemRemoved` should be called.

Add `TreeDataGridPresenterBase.UnrealizeOnRowRemoved` (with an override in  `TreeDataGridRowsPresenter`) and make `TreeDataGridPresenterBase.RecycleElementOnItemRemoved` call it.

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridCellsPresenter.cs

@@ -129,6 +129,14 @@ internal void UpdateSelection(ITreeDataGridSelectionInteraction? selection)
             }
         }
 
+        internal void UnrealizeOnRowRemoved()
+        {
+            if (RowIndex == -1)
+                throw new InvalidOperationException("Row is not realized.");
+            RowIndex = -1;
+            RecycleAllElementsOnItemRemoved();
+        }
+
         private ITreeDataGridSelectionInteraction? GetSelection()
         {
             return this.FindAncestorOfType<TreeDataGrid>()?.SelectionInteraction;

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridPresenterBase.cs

@@ -177,6 +177,15 @@ public IEnumerable<Control> GetRealizedElements()
 
         internal void RecycleAllElements() => _realizedElements?.RecycleAllElements(_recycleElement);
 
+        internal void RecycleAllElementsOnItemRemoved()
+        {
+            _realizedElements?.ItemsRemoved(
+                _realizedElements.FirstIndex,
+                _realizedElements.Count,
+                _updateElementIndex, 
+                _recycleElementOnItemRemoved);
+        }
+
         protected virtual Rect ArrangeElement(int index, Control element, Rect rect)
         {
             element.Arrange(rect);
@@ -427,6 +436,11 @@ protected virtual void OnEffectiveViewportChanged(object? sender, EffectiveViewp
             }
         }
 
+        protected virtual void UnrealizeElementOnItemRemoved(Control element)
+        {
+            UnrealizeElement(element);
+        }
+
         private void RealizeElements(
             IReadOnlyList<TItem> items,
             Size availableSize,
@@ -643,7 +657,7 @@ private void RecycleElement(Control element, int index)
 
         private void RecycleElementOnItemRemoved(Control element)
         {
-            UnrealizeElement(element);
+            UnrealizeElementOnItemRemoved(element);
             element.IsVisible = false;
             ElementFactory!.RecycleElement(element);
         }

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridRow.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Text;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Models.TreeDataGrid;
 using Avalonia.Controls.Selection;
@@ -106,7 +107,6 @@ public void Unrealize()
             IsSelected = false;
             CellsPresenter?.Unrealize();
         }
-
         protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
         {
             _treeDataGrid = this.FindLogicalAncestorOfType<TreeDataGrid>();
@@ -177,5 +177,14 @@ internal void UpdateSelection(ITreeDataGridSelectionInteraction? selection)
             IsSelected = selection?.IsRowSelected(RowIndex) ?? false;
             CellsPresenter?.UpdateSelection(selection);
         }
+
+        public void UnrealizeOnItemRemoved()
+        {
+            _treeDataGrid?.RaiseRowClearing(this, RowIndex);
+            RowIndex = -1;
+            DataContext = null;
+            IsSelected = false;
+            CellsPresenter?.UnrealizeOnRowRemoved();
+        }
     }
 }

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridRowsPresenter.cs

@@ -51,6 +51,12 @@ protected override void UnrealizeElement(Control element)
             ChildIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element, ((TreeDataGridRow)element).RowIndex));
         }
 
+        protected override void UnrealizeElementOnItemRemoved(Control element)
+        {
+            ((TreeDataGridRow)element).UnrealizeOnItemRemoved();
+            ChildIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element, ((TreeDataGridRow)element).RowIndex));
+        }
+
         protected override Size ArrangeOverride(Size finalSize)
         {
             Columns?.CommitActualWidths();