ux: use AvaloniaEdit.TextEditor to display release update detail info

src/Views/SelfUpdate.axaml

@@ -52,13 +52,13 @@
                 </Border>
               </StackPanel>
 
-              <TextBox IsReadOnly="True"
-                       TextWrapping="Wrap"
-                       ScrollViewer.VerticalScrollBarVisibility="Auto"
-                       MaxWidth="500" MaxHeight="400"
-                       Margin="0,8" Padding="0"
-                       VerticalContentAlignment="Top"
-                       Text="{Binding Body}"/>
+              <Border Width="500" MaxHeight="400"
+                      Margin="0,8"
+                      Background="{DynamicResource Brush.Contents}"
+                      BorderThickness="1"
+                      BorderBrush="{DynamicResource Brush.Border2}">
+                <v:UpdateInfoView/>
+              </Border>
 
               <StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
                 <Button Classes="flat primary"

src/Views/SelfUpdate.axaml.cs

@@ -1,8 +1,70 @@
+using System;
+
+using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
 using Avalonia.Interactivity;
 
+using AvaloniaEdit.Document;
+using AvaloniaEdit.Editing;
+using AvaloniaEdit.TextMate;
+using AvaloniaEdit;
+
 namespace SourceGit.Views
 {
+    public class UpdateInfoView : TextEditor
+    {
+        protected override Type StyleKeyOverride => typeof(TextEditor);
+
+        public UpdateInfoView() : base(new TextArea(), new TextDocument())
+        {
+            IsReadOnly = true;
+            ShowLineNumbers = false;
+            WordWrap = true;
+            HorizontalScrollBarVisibility = ScrollBarVisibility.Disabled;
+            VerticalScrollBarVisibility = ScrollBarVisibility.Auto;
+
+            TextArea.TextView.Margin = new Thickness(4, 0);
+            TextArea.TextView.Options.EnableHyperlinks = false;
+            TextArea.TextView.Options.EnableEmailHyperlinks = false;
+
+        }
+
+        protected override void OnLoaded(RoutedEventArgs e)
+        {
+            base.OnLoaded(e);
+
+            if (_textMate == null)
+            {
+                _textMate = Models.TextMateHelper.CreateForEditor(this);
+                Models.TextMateHelper.SetGrammarByFileName(_textMate, "README.md");
+            }
+        }
+
+        protected override void OnUnloaded(RoutedEventArgs e)
+        {
+            base.OnUnloaded(e);
+
+            if (_textMate != null)
+            {
+                _textMate.Dispose();
+                _textMate = null;
+            }
+
+            GC.Collect();
+        }
+
+        protected override void OnDataContextChanged(EventArgs e)
+        {
+            base.OnDataContextChanged(e);
+
+            if (DataContext is Models.Version ver)
+                Text = ver.Body;
+        }
+
+        private TextMate.Installation _textMate = null;
+    }
+
     public partial class SelfUpdate : ChromelessWindow
     {
         public SelfUpdate()