Update selected context when selected index changes

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -358,7 +358,7 @@ namespace Avalonia.Controls.Primitives
             {
                 if ((container.ContainerControl as ISelectable)?.IsSelected == true)
                 {
-                    if (SelectedIndex == -1)
+                    if (SelectionMode.HasFlag(SelectionMode.Single))
                     {
                         SelectedIndex = container.Index;
                     }

src/Avalonia.Controls/TabControl.cs

@@ -70,6 +70,7 @@ namespace Avalonia.Controls
             SelectionModeProperty.OverrideDefaultValue<TabControl>(SelectionMode.AlwaysSelected);
             ItemsPanelProperty.OverrideDefaultValue<TabControl>(DefaultPanel);
             AffectsMeasure<TabControl>(TabStripPlacementProperty);
+            SelectedIndexProperty.Changed.AddClassHandler<TabControl>((x, e) => x.UpdateSelectedContent(e));
         }
 
         /// <summary>
@@ -145,6 +146,64 @@ namespace Avalonia.Controls
             return RegisterContentPresenter(presenter);
         }
 
+        private void UpdateSelectedContent(AvaloniaPropertyChangedEventArgs e)
+        {
+            var index = (int)e.NewValue;
+
+            if (index == -1)
+            {
+                SelectedContentTemplate = null;
+
+                SelectedContent = null;
+
+                return;
+            }
+
+            var container = (TabItem)ItemContainerGenerator.ContainerFromIndex(index);
+
+            if (container == null)
+            {
+                if (Items is AvaloniaList<object> items)
+                {
+                    container = items[index] as TabItem;
+                }
+            }
+
+            if (container == null)
+            {
+                return;
+            }
+
+            UpdateSelectedContent(container);
+        }
+
+        private void UpdateSelectedContent(TabItem item)
+        {
+            if (SelectedContentTemplate != item.ContentTemplate)
+            {
+                SelectedContentTemplate = item.ContentTemplate;
+            }
+
+            if (SelectedContent != item.Content)
+            {
+                SelectedContent = item.Content;
+            }
+        }
+
+        protected override Size MeasureOverride(Size availableSize)
+        {
+            var size = base.MeasureOverride(availableSize);
+
+            if (SelectedIndex != -1 && SelectedContent == null)
+            {
+                var container = (TabItem)ItemContainerGenerator.ContainerFromIndex(SelectedIndex);
+
+                UpdateSelectedContent(container);
+            }
+
+            return size;
+        }
+
         /// <summary>
         /// Called when an <see cref="IContentPresenter"/> is registered with the control.
         /// </summary>

src/Avalonia.Controls/TabItem.cs

@@ -30,7 +30,6 @@ namespace Avalonia.Controls
         {
             SelectableMixin.Attach<TabItem>(IsSelectedProperty);
             FocusableProperty.OverrideDefaultValue(typeof(TabItem), true);
-            IsSelectedProperty.Changed.AddClassHandler<TabItem>((x, e) => x.UpdateSelectedContent(e));
             DataContextProperty.Changed.AddClassHandler<TabItem>((x, e) => x.UpdateHeader(e));
         }
 
@@ -83,23 +82,5 @@ namespace Avalonia.Controls
                 }
             }          
         }
-
-        private void UpdateSelectedContent(AvaloniaPropertyChangedEventArgs e)
-        {
-            if (!IsSelected)
-            {
-                return;
-            }
-
-            if (ParentTabControl.SelectedContentTemplate != ContentTemplate)
-            {
-                ParentTabControl.SelectedContentTemplate = ContentTemplate;
-            }
-
-            if (ParentTabControl.SelectedContent != Content)
-            {
-                ParentTabControl.SelectedContent = Content;
-            }
-        }
     }
 }