add a contextmenu to resources.

src/Avalonia.Themes.Default/TextBox.xaml

@@ -1,4 +1,11 @@
 <Styles xmlns="https://github.com/avaloniaui" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+  <Styles.Resources>
+    <ContextMenu x:Key="DefaultTextBoxContextMenu">
+      <MenuItem Header="Cut" Command="{Binding $parent[TextBox].Cut}" IsEnabled="{Binding $parent[TextBox].CanCut}" InputGesture="{x:Static TextBox.CutGesture}" />
+      <MenuItem Header="Copy" Command="{Binding $parent[TextBox].Copy}" IsEnabled="{Binding $parent[TextBox].CanCopy}" InputGesture="{x:Static TextBox.CopyGesture}"/>
+      <MenuItem Header="Paste" Command="{Binding $parent[TextBox].Paste}" IsEnabled="{Binding $parent[TextBox].CanPaste}" InputGesture="{x:Static TextBox.PasteGesture}"/>
+    </ContextMenu>
+  </Styles.Resources>
   <Style Selector="TextBox">
     <Setter Property="CaretBrush" Value="{DynamicResource ThemeForegroundBrush}" />
     <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>