Fix multiselection

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -358,17 +358,17 @@ namespace Avalonia.Controls.Primitives
             {
                 if ((container.ContainerControl as ISelectable)?.IsSelected == true)
                 {
-                    if (SelectionMode.HasFlag(SelectionMode.Single))
-                    {
-                        SelectedIndex = container.Index;
-                    }
-                    else
+                    if (SelectionMode.HasFlag(SelectionMode.Multiple))
                     {
                         if (_selection.Add(container.Index))
                         {
                             resetSelectedItems = true;
                         }
                     }
+                    else
+                    {
+                        SelectedIndex = container.Index;
+                    }
 
                     MarkContainerSelected(container.ContainerControl, true);
                 }

src/Avalonia.Controls/TabControl.cs

@@ -146,6 +146,25 @@ namespace Avalonia.Controls
             return RegisterContentPresenter(presenter);
         }
 
+        protected override void OnContainersMaterialized(ItemContainerEventArgs e)
+        {
+            base.OnContainersMaterialized(e);
+
+            if (SelectedContent != null || SelectedIndex == -1)
+            {
+                return;
+            }
+
+            var container = (TabItem)ItemContainerGenerator.ContainerFromIndex(SelectedIndex);
+
+            if (container == null)
+            {
+                return;
+            }
+
+            UpdateSelectedContent(container);
+        }
+
         private void UpdateSelectedContent(AvaloniaPropertyChangedEventArgs e)
         {
             var index = (int)e.NewValue;
@@ -190,20 +209,6 @@ namespace Avalonia.Controls
             }
         }
 
-        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>

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

@@ -1080,6 +1080,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             var target = new TestSelector
             {
                 Items = items,
+                SelectionMode = SelectionMode.Multiple,
                 Template = Template(),
             };
 

tests/Avalonia.Controls.UnitTests/TabControlTests.cs

@@ -63,9 +63,7 @@ namespace Avalonia.Controls.UnitTests
 
             target.Items = items;
 
-            target.ApplyTemplate();
-
-            target.Measure(Size.Infinity);
+            ApplyTemplate(target);
 
             Assert.Equal(secondContent, target.SelectedContent);
         }