Fix failing selection tests.

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -1232,16 +1232,18 @@ namespace Avalonia.Controls.Primitives
                     Selection = state.Selection.Value;
                 }
 
-                if (state.SelectedItems.HasValue)
+                if (_selection is InternalSelectionModel s)
                 {
-                    SelectedItems = state.SelectedItems.Value;
+                    s.Update(ItemsView.Source, state.SelectedItems);
                 }
-
-                Selection.Source = ItemsView.Source;
-
-                if (ItemsView.Count == 0)
+                else
                 {
-                    Selection.Clear();
+                    if (state.SelectedItems.HasValue)
+                    {
+                        SelectedItems = state.SelectedItems.Value;
+                    }
+
+                    Selection.Source = ItemsView.Source;
                 }
 
                 if (state.SelectedValue.HasValue)

src/Avalonia.Controls/Selection/InternalSelectionModel.cs

@@ -5,6 +5,7 @@ using System.Collections.Specialized;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Avalonia.Collections;
+using Avalonia.Data;
 
 namespace Avalonia.Controls.Selection
 {
@@ -13,6 +14,7 @@ namespace Avalonia.Controls.Selection
         private IList? _writableSelectedItems;
         private int _ignoreModelChanges;
         private bool _ignoreSelectedItemsChanges;
+        private bool _skipSyncFromSelectedItems;
         private bool _isResetting;
 
         public InternalSelectionModel()
@@ -60,6 +62,29 @@ namespace Avalonia.Controls.Selection
             }
         }
 
+        internal void Update(IEnumerable? source, Optional<IList?> selectedItems)
+        {
+            var previousSource = Source;
+            var previousWritableSelectedItems = _writableSelectedItems;
+
+            try
+            {
+                _skipSyncFromSelectedItems = true;
+                SetSource(source);
+                if (selectedItems.HasValue)
+                    WritableSelectedItems = selectedItems.Value;
+            }
+            finally 
+            { 
+                _skipSyncFromSelectedItems = false;
+            }
+
+            // We skipped the sync from WritableSelectedItems before; do it now that both
+            // the source and WritableSelectedItems are updated.
+            if (previousSource != Source || previousWritableSelectedItems != _writableSelectedItems)
+                SyncFromSelectedItems();
+        }
+
         private protected override void SetSource(IEnumerable? value)
         {
             if (Source == value)
@@ -121,7 +146,7 @@ namespace Avalonia.Controls.Selection
 
         private void SyncFromSelectedItems()
         {
-            if (Source is null || _writableSelectedItems is null)
+            if (_skipSyncFromSelectedItems || Source is null || _writableSelectedItems is null)
             {
                 return;
             }