Set SelectedIndex/Item on (Un)SelectAll().

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -495,6 +495,8 @@ namespace Avalonia.Controls.Primitives
                     _selection.Add(i);
                 }
 
+                UpdateSelectedItem(0, false);
+
                 foreach (var container in ItemContainerGenerator.Containers)
                 {
                     MarkItemSelected(container.Index, true);
@@ -507,20 +509,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Deselects all items in the control.
         /// </summary>
-        protected void UnselectAll()
-        {
-            UpdateSelectedItems(() =>
-            {
-                _selection.Clear();
-
-                foreach (var container in ItemContainerGenerator.Containers)
-                {
-                    MarkItemSelected(container.Index, false);
-                }
-
-                SelectedItems.Clear();
-            });
-        }
+        protected void UnselectAll() => UpdateSelectedItem(-1);
 
         /// <summary>
         /// Updates the selection for an item based on user interaction.

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests_Multiple.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Linq;
 using Avalonia.Collections;
 using Avalonia.Controls.Presenters;
@@ -783,6 +784,45 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(new[] { "Foo", "Bar", "Foo", "Bar" }, target.SelectedItems);
         }
 
+        [Fact]
+        public void SelectAll_Sets_SelectedIndex_And_SelectedItem()
+        {
+            var target = new TestSelector
+            {
+                Template = Template(),
+                Items = new[] { "Foo", "Bar", "Baz" },
+                SelectionMode = SelectionMode.Multiple,
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            target.SelectAll();
+
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Equal("Foo", target.SelectedItem);
+        }
+
+        [Fact]
+        public void UnselectAll_Clears_SelectedIndex_And_SelectedItem()
+        {
+            var target = new TestSelector
+            {
+                Template = Template(),
+                Items = new[] { "Foo", "Bar", "Baz" },
+                SelectionMode = SelectionMode.Multiple,
+                SelectedIndex = 0,
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            target.UnselectAll();
+
+            Assert.Equal(-1, target.SelectedIndex);
+            Assert.Equal(null, target.SelectedItem);
+        }
+
         [Fact]
         public void SelectAll_Handles_Duplicate_Items()
         {
@@ -836,9 +876,8 @@ namespace Avalonia.Controls.UnitTests.Primitives
             }
 
             public new void SelectAll() => base.SelectAll();
-
+            public new void UnselectAll() => base.UnselectAll();
             public void SelectRange(int index) => UpdateSelection(index, true, true);
-
             public void Toggle(int index) => UpdateSelection(index, true, false, true);
         }