From 55e9037383df98654870186db13031ab50f27bca Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Mon, 7 Oct 2019 10:39:50 +0200
Subject: [PATCH 1/2] Added failing test for #2980.

---
 .../TreeViewTests.cs                          | 41 +++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs b/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs
index 6c9f0f4f57..a91b7a0701 100644
--- a/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs
@@ -14,6 +14,7 @@ using Avalonia.Input;
 using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
 using Avalonia.LogicalTree;
+using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -892,6 +893,46 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(2, GetItem(target, 0, 1, 0).Level);
         }
 
+        [Fact]
+        public void Auto_Expanding_In_Style_Should_Not_Break_Range_Selection()
+        {
+            /// Issue #2980.
+            using (UnitTestApplication.Start(TestServices.RealStyler))
+            {
+                var target = new DerivedTreeView
+                {
+                    Template = CreateTreeViewTemplate(),
+                    SelectionMode = SelectionMode.Multiple,
+                    Items = new List<Node>
+                {
+                    new Node { Value = "Root1", },
+                    new Node { Value = "Root2", },
+                },
+                };
+
+                var visualRoot = new TestRoot
+                {
+                    Styles =
+                    {
+                        new Style(x => x.OfType<TreeViewItem>())
+                        {
+                            Setters =
+                            {
+                                new Setter(TreeViewItem.IsExpandedProperty, true),
+                            },
+                        },
+                    },
+                    Child = target,
+                };
+
+                CreateNodeDataTemplate(target);
+                ApplyTemplates(target);
+
+                _mouse.Click(GetItem(target, 0));
+                _mouse.Click(GetItem(target, 1), modifiers: InputModifiers.Shift);
+            }
+        }
+
         private void ApplyTemplates(TreeView tree)
         {
             tree.ApplyTemplate();

From 75fdd0d3432a033c0b8c93e8d583754c58a6a2e3 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Mon, 7 Oct 2019 10:41:44 +0200
Subject: [PATCH 2/2] Check for empty items in GetContainerInDirection.

Don't try to navigate into `TreeViewItem`s with no children: this causes a out-of-range exception.

Fixes #2980
---
 src/Avalonia.Controls/TreeView.cs | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/Avalonia.Controls/TreeView.cs b/src/Avalonia.Controls/TreeView.cs
index cc47365214..8907137ecb 100644
--- a/src/Avalonia.Controls/TreeView.cs
+++ b/src/Avalonia.Controls/TreeView.cs
@@ -1,3 +1,4 @@
+
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
@@ -470,7 +471,7 @@ namespace Avalonia.Controls
                     if (index > 0)
                     {
                         var previous = (TreeViewItem)parentGenerator.ContainerFromIndex(index - 1);
-                        result = previous.IsExpanded ?
+                        result = previous.IsExpanded && previous.ItemCount > 0 ?
                             (TreeViewItem)previous.ItemContainerGenerator.ContainerFromIndex(previous.ItemCount - 1) :
                             previous;
                     }
@@ -482,7 +483,7 @@ namespace Avalonia.Controls
                     break;
 
                 case NavigationDirection.Down:
-                    if (from.IsExpanded && intoChildren)
+                    if (from.IsExpanded && intoChildren && from.ItemCount > 0)
                     {
                         result = (TreeViewItem)from.ItemContainerGenerator.ContainerFromIndex(0);
                     }