From 831082a364961c230e9faff704e55945c4e91a18 Mon Sep 17 00:00:00 2001
From: Lachlan Ennis <2433737+elachlan@users.noreply.github.com>
Date: Mon, 18 Dec 2023 10:10:26 +1000
Subject: [PATCH] Fixes #8768

---
 .../LocalAppContextSwitches.cs                   |  8 ++++++++
 .../Windows/Forms/Controls/TreeView/TreeNode.cs  | 16 ++++++++++------
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/src/System.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs b/src/System.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs
index 858899fcb56..f3d3212281f 100644
--- a/src/System.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs
@@ -21,6 +21,7 @@ internal static partial class LocalAppContextSwitches
     internal const string TrackBarModernRenderingSwitchName = "System.Windows.Forms.TrackBarModernRendering";
     private const string DoNotCatchUnhandledExceptionsSwitchName = "System.Windows.Forms.DoNotCatchUnhandledExceptions";
     internal const string DataGridViewUIAStartRowCountAtZeroSwitchName = "System.Windows.Forms.DataGridViewUIAStartRowCountAtZero";
+    internal const string TreeNodeDoNotUseFixedIndexWithSortedTreeViewSwitchName = "System.Windows.Forms.TreeNodeDoNotUseFixedIndexWithSortedTreeView";
 
     private static int s_scaleTopLevelFormMinMaxSizeForDpi;
     private static int s_anchorLayoutV2;
@@ -28,6 +29,7 @@ internal static partial class LocalAppContextSwitches
     private static int s_trackBarModernRendering;
     private static int s_doNotCatchUnhandledExceptions;
     private static int s_dataGridViewUIAStartRowCountAtZero;
+    private static int s_treeNodeDoNotUseFixedIndexWithSortedTreeView;
 
     private static FrameworkName? s_targetFrameworkName;
 
@@ -165,5 +167,11 @@ public static bool DataGridViewUIAStartRowCountAtZero
         get => GetCachedSwitchValue(DataGridViewUIAStartRowCountAtZeroSwitchName, ref s_dataGridViewUIAStartRowCountAtZero);
     }
 
+    public static bool TreeNodeDoNotUseFixedIndexWithSortedTreeView
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        get => GetCachedSwitchValue(TreeNodeDoNotUseFixedIndexWithSortedTreeViewSwitchName, ref s_treeNodeDoNotUseFixedIndexWithSortedTreeView);
+    }
+
     internal static void SetDataGridViewUIAStartRowCountAtZero(bool value) => s_dataGridViewUIAStartRowCountAtZero = value ? 1 : 0;
 }
diff --git a/src/System.Windows.Forms/src/System/Windows/Forms/Controls/TreeView/TreeNode.cs b/src/System.Windows.Forms/src/System/Windows/Forms/Controls/TreeView/TreeNode.cs
index d8db329a18e..aabdd7327e1 100644
--- a/src/System.Windows.Forms/src/System/Windows/Forms/Controls/TreeView/TreeNode.cs
+++ b/src/System.Windows.Forms/src/System/Windows/Forms/Controls/TreeView/TreeNode.cs
@@ -9,6 +9,7 @@
 using System.Runtime.InteropServices;
 using System.Runtime.Serialization;
 using System.Text;
+using System.Windows.Forms.Primitives;
 
 namespace System.Windows.Forms;
 
@@ -815,17 +816,20 @@ public TreeNodeCollection Nodes
             }
 
             // fixedIndex is used for perf. optimization in case of adding big ranges of nodes
-            int currentInd = _index;
-            int fixedInd = _parent.Nodes.FixedIndex;
+            int currentIndex = _index;
+            int fixedIndex = _parent.Nodes.FixedIndex;
 
-            if (fixedInd > 0)
+            if (fixedIndex > 0)
             {
-                currentInd = fixedInd;
+                if (!LocalAppContextSwitches.TreeNodeDoNotUseFixedIndexWithSortedTreeView || TreeView is null || TreeView.Sorted)
+                {
+                    currentIndex = fixedIndex;
+                }
             }
 
-            if (currentInd > 0 && currentInd <= _parent.Nodes.Count)
+            if (currentIndex > 0 && currentIndex <= _parent.Nodes.Count)
             {
-                return _parent.Nodes[currentInd - 1];
+                return _parent.Nodes[currentIndex - 1];
             }
             else
             {