Fix vertical caret navigation caret update

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using Avalonia.Data;
 using Avalonia.Media;
 using Avalonia.Media.TextFormatting;
 using Avalonia.Metadata;
@@ -17,7 +18,7 @@ namespace Avalonia.Controls.Presenters
         public static readonly DirectProperty<TextPresenter, int> CaretIndexProperty =
             TextBox.CaretIndexProperty.AddOwner<TextPresenter>(
                 o => o.CaretIndex,
-                (o, v) => o.CaretIndex = v);
+                (o, v) => o.CaretIndex = v, defaultBindingMode: BindingMode.TwoWay);
 
         public static readonly StyledProperty<bool> RevealPasswordProperty =
             AvaloniaProperty.Register<TextPresenter, bool>(nameof(RevealPassword));

src/Avalonia.Controls/TextBox.cs

@@ -939,6 +939,10 @@ namespace Avalonia.Controls
                         {
                             SelectionEnd = _presenter.CaretIndex;
                         }
+                        else
+                        {
+                            SelectionStart = SelectionEnd = _presenter.CaretIndex;
+                        }
                         
                         break;
                     }
@@ -957,6 +961,10 @@ namespace Avalonia.Controls
                         {
                             SelectionEnd = _presenter.CaretIndex;
                         }
+                        else
+                        {
+                            SelectionStart = SelectionEnd = _presenter.CaretIndex;
+                        }
                         
                         break;
                     }