use static resource for the context menus in textbox templates.

src/Avalonia.Themes.Default/TextBox.xaml

@@ -14,7 +14,7 @@
     <Setter Property="SelectionBrush" Value="{DynamicResource HighlightBrush}"/>
     <Setter Property="SelectionForegroundBrush" Value="{DynamicResource HighlightForegroundBrush}"/>
     <Setter Property="Padding" Value="4"/>
-    <Setter Property="ContextMenu" Value="{DynamicResource DefaultTextBoxContextMenu}" />
+    <Setter Property="ContextMenu" Value="{StaticResource DefaultTextBoxContextMenu}" />
     <Setter Property="Template">
       <ControlTemplate>
         <Border Name="border"

src/Avalonia.Themes.Fluent/TextBox.xaml

@@ -33,7 +33,7 @@
     <Setter Property="MinWidth" Value="{DynamicResource TextControlThemeMinWidth}" />
     <Setter Property="Padding" Value="{DynamicResource TextControlThemePadding}" />
     <Setter Property="FocusAdorner" Value="{x:Null}" />
-    <Setter Property="ContextMenu" Value="{DynamicResource DefaultTextBoxContextMenu}" />
+    <Setter Property="ContextMenu" Value="{StaticResource DefaultTextBoxContextMenu}" />
     <Setter Property="Template">
       <ControlTemplate>
         <DockPanel>