From fcaa250c72cbbb691911456a1be0691663e41c63 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Mon, 3 Feb 2020 13:52:43 +0100
Subject: [PATCH] Ported fix and test from WinUI.

https://github.com/microsoft/microsoft-ui-xaml/pull/1922
---
 .../Utils/SelectionTreeHelper.cs              | 18 +++++++----
 .../SelectionModelTests.cs                    | 31 +++++++++++++++++++
 2 files changed, 43 insertions(+), 6 deletions(-)

diff --git a/src/Avalonia.Controls/Utils/SelectionTreeHelper.cs b/src/Avalonia.Controls/Utils/SelectionTreeHelper.cs
index 93102a7b5b..430ecabbb8 100644
--- a/src/Avalonia.Controls/Utils/SelectionTreeHelper.cs
+++ b/src/Avalonia.Controls/Utils/SelectionTreeHelper.cs
@@ -132,15 +132,21 @@ namespace Avalonia.Controls.Utils
 
         private static bool IsSubSet(IndexPath path, IndexPath subset)
         {
-            bool isSubset = true;
-            for (int i = 0; i < subset.GetSize(); i++)
+            var subsetSize = subset.GetSize();
+            if (path.GetSize() < subsetSize)
             {
-                isSubset = path.GetAt(i) == subset.GetAt(i);
-                if (!isSubset)
-                    break;
+                return false;
+            }
+
+            for (int i = 0; i < subsetSize; i++)
+            {
+                if (path.GetAt(i) != subset.GetAt(i))
+                {
+                    return false;
+                }
             }
 
-            return isSubset;
+            return true;
         }
 
         private static IndexPath StartPath(IndexPath path, int length)
diff --git a/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs b/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs
index 6c3137c636..208d85d8fd 100644
--- a/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs
@@ -898,6 +898,37 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
+        [Fact]
+        public void SelectRangeRegressionTest()
+        {
+            RunOnUIThread.Execute(() =>
+            {
+                var selectionModel = new SelectionModel()
+                {
+                    Source = CreateNestedData(1, 2, 3)
+                };
+
+                // length of start smaller than end used to cause an out of range error.
+                selectionModel.SelectRange(IndexPath.CreateFrom(0), IndexPath.CreateFrom(1, 1));
+
+                ValidateSelection(selectionModel,
+                   new List<IndexPath>()
+                   {
+                       Path(0, 0),
+                       Path(0, 1),
+                       Path(0, 2),
+                       Path(0),
+                       Path(1, 0),
+                       Path(1, 1)
+                   },
+                   new List<IndexPath>()
+                   {
+                       Path(),
+                       Path(1)
+                   },
+                   1 /* selectedInnerNodes */);
+            });
+        }
 
         [Fact]
         public void Disposing_Unhooks_CollectionChanged_Handlers()