From f9a6f5b1ece28eeb4c584b6928fae493a0dd7394 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Wed, 21 Apr 2021 09:20:12 +0200
Subject: [PATCH 1/3] Create failing test for SelectedItems binding.

---
 .../ListBoxTests.cs                           | 28 +++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/tests/Avalonia.Controls.UnitTests/ListBoxTests.cs b/tests/Avalonia.Controls.UnitTests/ListBoxTests.cs
index 963bba7c83..7c57e22933 100644
--- a/tests/Avalonia.Controls.UnitTests/ListBoxTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/ListBoxTests.cs
@@ -1,3 +1,4 @@
+using System.Collections.ObjectModel;
 using System.Linq;
 using System.Reactive.Subjects;
 using Avalonia.Collections;
@@ -457,6 +458,33 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Initial_Binding_Of_SelectedItems_Should_Not_Cause_Write_To_SelectedItems()
+        {
+            var target = new ListBox
+            {
+                [!ListBox.ItemsProperty] = new Binding("Items"),
+                [!ListBox.SelectedItemsProperty] = new Binding("SelectedItems"),
+            };
+
+            var viewModel = new
+            {
+                Items = new[] { "Foo", "Bar", "Baz " },
+                SelectedItems = new ObservableCollection<string> { "Bar" },
+            };
+
+            var raised = 0;
+
+            viewModel.SelectedItems.CollectionChanged += (s, e) => ++raised;
+
+            target.DataContext = viewModel;
+
+            Assert.Equal(0, raised);
+            Assert.Equal(new[] { "Bar" }, viewModel.SelectedItems);
+            Assert.Equal(new[] { "Bar" }, target.SelectedItems);
+            Assert.Equal(new[] { "Bar" }, target.Selection.SelectedItems);
+        }
+
         private FuncControlTemplate ListBoxTemplate()
         {
             return new FuncControlTemplate<ListBox>((parent, scope) =>

From 67a1e0921ca68e578ba44289f3a0ddac4d837d6f Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Wed, 21 Apr 2021 10:00:21 +0200
Subject: [PATCH 2/3] Don't update SelectedItems when changing source.

---
 .../Selection/InternalSelectionModel.cs       | 33 +++++++++++++++----
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/src/Avalonia.Controls/Selection/InternalSelectionModel.cs b/src/Avalonia.Controls/Selection/InternalSelectionModel.cs
index c9d91b1ae6..d0da2f7ce2 100644
--- a/src/Avalonia.Controls/Selection/InternalSelectionModel.cs
+++ b/src/Avalonia.Controls/Selection/InternalSelectionModel.cs
@@ -80,10 +80,12 @@ namespace Avalonia.Controls.Selection
             try
             {
                 _ignoreSelectedItemsChanges = true;
+                ++_ignoreModelChanges;
                 base.SetSource(value);
             }
             finally
             {
+                --_ignoreModelChanges;
                 _ignoreSelectedItemsChanges = false;
             }
 
@@ -93,17 +95,14 @@ namespace Avalonia.Controls.Selection
             }
             else
             {
-                foreach (var i in oldSelection)
-                {
-                    var index = ItemsView!.IndexOf(i);
-                    Select(index);
-                }
+                SyncFromSelectedItems();
             }
         }
 
         private void SyncToSelectedItems()
         {
-            if (_writableSelectedItems is object)
+            if (_writableSelectedItems is object &&
+                !SequenceEqual(_writableSelectedItems, base.SelectedItems))
             {
                 try
                 {
@@ -310,5 +309,27 @@ namespace Avalonia.Controls.Selection
 
             return -1;
         }
+
+        private static bool SequenceEqual(IList first, IReadOnlyList<object?> second)
+        {
+            if (first is IEnumerable<object?> e)
+            {
+                return e.SequenceEqual(second);
+            }
+
+            var comparer = EqualityComparer<object?>.Default;
+            var e1 = first.GetEnumerator();
+            using var e2 = second.GetEnumerator();
+
+            while (e1.MoveNext())
+            {
+                if (!(e2.MoveNext() && comparer.Equals(e1.Current, e2.Current)))
+                {
+                    return false;
+                }
+            }
+
+            return !e2.MoveNext();
+        }
     }
 }

From eeb7c8d921fba166276e0b24c4509ed864c7a378 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Wed, 21 Apr 2021 13:26:51 +0200
Subject: [PATCH 3/3] Reset _isResetting, d'oh.

---
 src/Avalonia.Controls/Selection/InternalSelectionModel.cs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/Avalonia.Controls/Selection/InternalSelectionModel.cs b/src/Avalonia.Controls/Selection/InternalSelectionModel.cs
index d0da2f7ce2..73c0184ebb 100644
--- a/src/Avalonia.Controls/Selection/InternalSelectionModel.cs
+++ b/src/Avalonia.Controls/Selection/InternalSelectionModel.cs
@@ -223,6 +223,7 @@ namespace Avalonia.Controls.Selection
             if (_isResetting)
             {
                 --_ignoreModelChanges;
+                _isResetting = false;
             }
         }