feat: add avalonia SelectableTextBlock style.

src/Avalonia/HandyControl_Avalonia/Themes/Styles/MergedStyles.axaml

@@ -53,6 +53,7 @@
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/ComboBox.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/Menu.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/ContextMenu.axaml" />
+                <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/SelectableTextBlock.axaml" />
             </ResourceDictionary.MergedDictionaries>
         </ResourceDictionary>
     </Styles.Resources>

src/Avalonia/HandyControl_Avalonia/Themes/Styles/SelectableTextBlock.axaml

@@ -0,0 +1,27 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal">
+
+    <MenuFlyout x:Key="SelectableTextBlockContextFlyout"
+                Placement="Bottom">
+        <MenuItem Header="Copy"
+                  Cursor="Arrow"
+                  Command="{Binding $parent[SelectableTextBlock].Copy}"
+                  IsEnabled="{Binding $parent[SelectableTextBlock].CanCopy}"
+                  InputGesture="{x:Static TextBox.CopyGesture}" />
+    </MenuFlyout>
+
+    <ControlTheme x:Key="{x:Type SelectableTextBlock}"
+                  TargetType="SelectableTextBlock">
+        <Setter Property="SelectionBrush"
+                Value="{DynamicResource HighlightBrush}" />
+
+        <Style Selector="^[IsEnabled=True]">
+            <Setter Property="Cursor"
+                    Value="IBeam" />
+            <Setter Property="ContextFlyout"
+                    Value="{StaticResource SelectableTextBlockContextFlyout}" />
+        </Style>
+    </ControlTheme>
+
+</ResourceDictionary>