Fix failing tests

src/Avalonia.Controls/TextBox.cs

@@ -305,7 +305,7 @@ namespace Avalonia.Controls
                     UpdateCommandStates();
                 }
                 
-                if (SelectionStart == SelectionEnd)
+                if (value == SelectionEnd)
                 {
                     CaretIndex = SelectionStart;
                 }
@@ -328,6 +328,11 @@ namespace Avalonia.Controls
                 {
                     UpdateCommandStates();
                 }
+
+                if (value == SelectionStart)
+                {
+                    CaretIndex = value;
+                }
             }
         }
 

tests/Avalonia.Controls.UnitTests/TextBoxTests.cs

@@ -136,6 +136,8 @@ namespace Avalonia.Controls.UnitTests
                     Template = CreateTemplate(),
                     Text = "1234"
                 };
+                
+                target.ApplyTemplate();
 
                 RaiseKeyEvent(target, Key.A, KeyModifiers.Control);
 
@@ -304,6 +306,7 @@ namespace Avalonia.Controls.UnitTests
 
                 textBox.SelectionStart = 2;
                 textBox.SelectionEnd = 2;
+                
                 Assert.Equal(2, textBox.CaretIndex);
             }
         }