diff --git a/src/Avalonia.Base/Threading/DispatcherPriority.cs b/src/Avalonia.Base/Threading/DispatcherPriority.cs
index ceda1c397f..a2b4b86bac 100644
--- a/src/Avalonia.Base/Threading/DispatcherPriority.cs
+++ b/src/Avalonia.Base/Threading/DispatcherPriority.cs
@@ -17,7 +17,7 @@ namespace Avalonia.Threading
         SystemIdle = 1,
 
         /// <summary>
-        /// The job will be processed when the application sis idle.
+        /// The job will be processed when the application is idle.
         /// </summary>
         ApplicationIdle = 2,
 
diff --git a/src/Avalonia.Controls/TreeViewItem.cs b/src/Avalonia.Controls/TreeViewItem.cs
index a224ceaadd..d3bd45d13c 100644
--- a/src/Avalonia.Controls/TreeViewItem.cs
+++ b/src/Avalonia.Controls/TreeViewItem.cs
@@ -51,6 +51,7 @@ namespace Avalonia.Controls
             SelectableMixin.Attach<TreeViewItem>(IsSelectedProperty);
             FocusableProperty.OverrideDefaultValue<TreeViewItem>(true);
             ItemsPanelProperty.OverrideDefaultValue<TreeViewItem>(DefaultPanel);
+            ParentProperty.Changed.AddClassHandler<TreeViewItem>((o, e) => o.OnParentChanged(e));
             RequestBringIntoViewEvent.AddClassHandler<TreeViewItem>((x, e) => x.OnRequestBringIntoView(e));
         }
 
@@ -179,5 +180,16 @@ namespace Avalonia.Controls
 
             return logical != null ? result : @default;
         }
+
+        private void OnParentChanged(AvaloniaPropertyChangedEventArgs e)
+        {
+            if (!((ILogical)this).IsAttachedToLogicalTree && e.NewValue is null)
+            {
+                // If we're not attached to the logical tree, then OnDetachedFromLogicalTree isn't going to be
+                // called when the item is removed. This results in the item not being removed from the index,
+                // causing #3551. In this case, update the index when Parent is changed to null.
+                ItemContainerGenerator.UpdateIndex();
+            }
+        }
     }
 }
diff --git a/src/Avalonia.Input/FocusManager.cs b/src/Avalonia.Input/FocusManager.cs
index bcae8a3c53..011ae6ce6b 100644
--- a/src/Avalonia.Input/FocusManager.cs
+++ b/src/Avalonia.Input/FocusManager.cs
@@ -53,11 +53,11 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="control">The control to focus.</param>
         /// <param name="method">The method by which focus was changed.</param>
-        /// <param name="modifiers">Any input modifiers active at the time of focus.</param>
+        /// <param name="keyModifiers">Any key modifiers active at the time of focus.</param>
         public void Focus(
             IInputElement control, 
             NavigationMethod method = NavigationMethod.Unspecified,
-            InputModifiers modifiers = InputModifiers.None)
+            KeyModifiers keyModifiers = KeyModifiers.None)
         {
             if (control != null)
             {
@@ -67,7 +67,7 @@ namespace Avalonia.Input
                 if (scope != null)
                 {
                     Scope = scope;
-                    SetFocusedElement(scope, control, method, modifiers);
+                    SetFocusedElement(scope, control, method, keyModifiers);
                 }
             }
             else if (Current != null)
@@ -95,7 +95,7 @@ namespace Avalonia.Input
         /// <param name="scope">The focus scope.</param>
         /// <param name="element">The element to focus. May be null.</param>
         /// <param name="method">The method by which focus was changed.</param>
-        /// <param name="modifiers">Any input modifiers active at the time of focus.</param>
+        /// <param name="keyModifiers">Any key modifiers active at the time of focus.</param>
         /// <remarks>
         /// If the specified scope is the current <see cref="Scope"/> then the keyboard focus
         /// will change.
@@ -104,7 +104,7 @@ namespace Avalonia.Input
             IFocusScope scope,
             IInputElement element,
             NavigationMethod method = NavigationMethod.Unspecified,
-            InputModifiers modifiers = InputModifiers.None)
+            KeyModifiers keyModifiers = KeyModifiers.None)
         {
             Contract.Requires<ArgumentNullException>(scope != null);
 
@@ -123,7 +123,7 @@ namespace Avalonia.Input
 
             if (Scope == scope)
             {
-                KeyboardDevice.Instance?.SetFocusedElement(element, method, modifiers);
+                KeyboardDevice.Instance?.SetFocusedElement(element, method, keyModifiers);
             }
         }
 
@@ -195,7 +195,7 @@ namespace Avalonia.Input
                 {
                     if (element is IInputElement inputElement && CanFocus(inputElement))
                     {
-                        Instance?.Focus(inputElement, NavigationMethod.Pointer, ev.InputModifiers);
+                        Instance?.Focus(inputElement, NavigationMethod.Pointer, ev.KeyModifiers);
 
                         break;
                     }
diff --git a/src/Avalonia.Input/IFocusManager.cs b/src/Avalonia.Input/IFocusManager.cs
index 84cd791ee0..9122cc428d 100644
--- a/src/Avalonia.Input/IFocusManager.cs
+++ b/src/Avalonia.Input/IFocusManager.cs
@@ -20,11 +20,11 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="control">The control to focus.</param>
         /// <param name="method">The method by which focus was changed.</param>
-        /// <param name="modifiers">Any input modifiers active at the time of focus.</param>
+        /// <param name="keyModifiers">Any key modifiers active at the time of focus.</param>
         void Focus(
-            IInputElement control, 
+            IInputElement control,
             NavigationMethod method = NavigationMethod.Unspecified,
-            InputModifiers modifiers = InputModifiers.None);
+            KeyModifiers keyModifiers = KeyModifiers.None);
 
         /// <summary>
         /// Notifies the focus manager of a change in focus scope.
diff --git a/src/Avalonia.Input/IKeyboardDevice.cs b/src/Avalonia.Input/IKeyboardDevice.cs
index 2725638b9a..ba7e0484ee 100644
--- a/src/Avalonia.Input/IKeyboardDevice.cs
+++ b/src/Avalonia.Input/IKeyboardDevice.cs
@@ -63,6 +63,6 @@ namespace Avalonia.Input
         void SetFocusedElement(
             IInputElement element, 
             NavigationMethod method,
-            InputModifiers modifiers);
+            KeyModifiers modifiers);
     }
 }
diff --git a/src/Avalonia.Input/IKeyboardNavigationHandler.cs b/src/Avalonia.Input/IKeyboardNavigationHandler.cs
index 4d0ae7e85d..88d00b3b50 100644
--- a/src/Avalonia.Input/IKeyboardNavigationHandler.cs
+++ b/src/Avalonia.Input/IKeyboardNavigationHandler.cs
@@ -19,10 +19,10 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="element">The current element.</param>
         /// <param name="direction">The direction to move.</param>
-        /// <param name="modifiers">Any input modifiers active at the time of focus.</param>
+        /// <param name="keyModifiers">Any key modifiers active at the time of focus.</param>
         void Move(
             IInputElement element, 
             NavigationDirection direction,
-            InputModifiers modifiers = InputModifiers.None);
+            KeyModifiers keyModifiers = KeyModifiers.None);
     }
-}
\ No newline at end of file
+}
diff --git a/src/Avalonia.Input/KeyboardDevice.cs b/src/Avalonia.Input/KeyboardDevice.cs
index 006a6b12d9..0321b0bdf3 100644
--- a/src/Avalonia.Input/KeyboardDevice.cs
+++ b/src/Avalonia.Input/KeyboardDevice.cs
@@ -35,7 +35,7 @@ namespace Avalonia.Input
         public void SetFocusedElement(
             IInputElement element, 
             NavigationMethod method,
-            InputModifiers modifiers)
+            KeyModifiers keyModifiers)
         {
             if (element != FocusedElement)
             {
@@ -53,7 +53,7 @@ namespace Avalonia.Input
                 {
                     RoutedEvent = InputElement.GotFocusEvent,
                     NavigationMethod = method,
-                    InputModifiers = modifiers,
+                    KeyModifiers = keyModifiers,
                 });
             }
         }
diff --git a/src/Avalonia.Input/KeyboardNavigationHandler.cs b/src/Avalonia.Input/KeyboardNavigationHandler.cs
index 323a225b50..c425eeeedb 100644
--- a/src/Avalonia.Input/KeyboardNavigationHandler.cs
+++ b/src/Avalonia.Input/KeyboardNavigationHandler.cs
@@ -91,11 +91,11 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="element">The current element.</param>
         /// <param name="direction">The direction to move.</param>
-        /// <param name="modifiers">Any input modifiers active at the time of focus.</param>
+        /// <param name="keyModifiers">Any key modifiers active at the time of focus.</param>
         public void Move(
             IInputElement element, 
             NavigationDirection direction,
-            InputModifiers modifiers = InputModifiers.None)
+            KeyModifiers keyModifiers = KeyModifiers.None)
         {
             Contract.Requires<ArgumentNullException>(element != null);
 
@@ -106,7 +106,7 @@ namespace Avalonia.Input
                 var method = direction == NavigationDirection.Next ||
                              direction == NavigationDirection.Previous ?
                              NavigationMethod.Tab : NavigationMethod.Directional;
-                FocusManager.Instance.Focus(next, method, modifiers);
+                FocusManager.Instance.Focus(next, method, keyModifiers);
             }
         }
 
@@ -123,7 +123,7 @@ namespace Avalonia.Input
             {
                 var direction = (e.KeyModifiers & KeyModifiers.Shift) == 0 ?
                     NavigationDirection.Next : NavigationDirection.Previous;
-                Move(current, direction, e.Modifiers);
+                Move(current, direction, e.KeyModifiers);
                 e.Handled = true;
             }
         }
diff --git a/src/Windows/Avalonia.Win32.Interop/WinForms/WinFormsAvaloniaControlHost.cs b/src/Windows/Avalonia.Win32.Interop/WinForms/WinFormsAvaloniaControlHost.cs
index fe626f4d38..abace92f08 100644
--- a/src/Windows/Avalonia.Win32.Interop/WinForms/WinFormsAvaloniaControlHost.cs
+++ b/src/Windows/Avalonia.Win32.Interop/WinForms/WinFormsAvaloniaControlHost.cs
@@ -45,7 +45,7 @@ namespace Avalonia.Win32.Embedding
                 focused = focused.VisualParent;
 
             if (focused == _root)
-                KeyboardDevice.Instance.SetFocusedElement(null, NavigationMethod.Unspecified, InputModifiers.None);
+                KeyboardDevice.Instance.SetFocusedElement(null, NavigationMethod.Unspecified, KeyModifiers.None);
         }
 
         private void PlatformImpl_LostFocus()
diff --git a/src/Windows/Avalonia.Win32/Input/WindowsKeyboardDevice.cs b/src/Windows/Avalonia.Win32/Input/WindowsKeyboardDevice.cs
index f1123e3958..1258bb0109 100644
--- a/src/Windows/Avalonia.Win32/Input/WindowsKeyboardDevice.cs
+++ b/src/Windows/Avalonia.Win32/Input/WindowsKeyboardDevice.cs
@@ -44,7 +44,7 @@ namespace Avalonia.Win32.Input
 
         public void WindowActivated(Window window)
         {
-            SetFocusedElement(window, NavigationMethod.Unspecified, InputModifiers.None);
+            SetFocusedElement(window, NavigationMethod.Unspecified, KeyModifiers.None);
         }
 
         public string StringFromVirtualKey(uint virtualKey)
diff --git a/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs b/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs
index 32b09e2c47..bd303a81cd 100644
--- a/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs
@@ -1002,6 +1002,35 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(1, child2Node.Presenter.Panel.Children.Count);
         }
 
+        [Fact]
+        public void Clearing_TreeView_Items_Clears_Index()
+        {
+            // Issue #3551
+            var tree = CreateTestTreeData();
+            var target = new TreeView
+            {
+                Template = CreateTreeViewTemplate(),
+                Items = tree,
+            };
+
+            var root = new TestRoot();
+            root.Child = target;
+
+            CreateNodeDataTemplate(target);
+            ApplyTemplates(target);
+
+            var rootNode = tree[0];
+            var container = (TreeViewItem)target.ItemContainerGenerator.Index.ContainerFromItem(rootNode);
+
+            Assert.NotNull(container);
+
+            root.Child = null;
+
+            tree.Clear();
+
+            Assert.Empty(target.ItemContainerGenerator.Index.Containers);
+        }
+
         private void ApplyTemplates(TreeView tree)
         {
             tree.ApplyTemplate();
diff --git a/tests/Avalonia.Input.UnitTests/KeyboardDeviceTests.cs b/tests/Avalonia.Input.UnitTests/KeyboardDeviceTests.cs
index 3c8e800fca..df0a077c7f 100644
--- a/tests/Avalonia.Input.UnitTests/KeyboardDeviceTests.cs
+++ b/tests/Avalonia.Input.UnitTests/KeyboardDeviceTests.cs
@@ -35,7 +35,7 @@ namespace Avalonia.Input.UnitTests
             target.SetFocusedElement(
                 focused.Object,
                 NavigationMethod.Unspecified,
-                InputModifiers.None);
+                KeyModifiers.None);
 
             target.ProcessRawEvent(
                 new RawKeyEventArgs(
@@ -75,7 +75,7 @@ namespace Avalonia.Input.UnitTests
             target.SetFocusedElement(
                 focused.Object,
                 NavigationMethod.Unspecified,
-                InputModifiers.None);
+                KeyModifiers.None);
 
             target.ProcessRawEvent(
                 new RawTextInputEventArgs(