Fix fileviewer ShowLineNumbers setting that was not honored

and thus line numbers were displayed in blame even if explicitly set to be hidden

That was the case since 456ea65

See 456ea65#diff-555178f3fd75ec9d4f3aebcef1f48882R128

The strategy adopted is to not set the value by default so the setting is determined by the content (actual behavior)
but if explicitly set, the value is used.

Add padding and color it on white to add a margin when line numbers are hidden

GitUI/Editor/FileViewer.cs

@@ -191,10 +191,10 @@ public bool IsReadOnly
             set => internalFileViewer.IsReadOnly = value;
         }
 
-        [DefaultValue(true)]
+        [DefaultValue(null)]
         [Description("If true line numbers are shown in the textarea")]
         [Category("Appearance")]
-        public bool ShowLineNumbers
+        public bool? ShowLineNumbers
         {
             get => internalFileViewer.ShowLineNumbers;
             set => internalFileViewer.ShowLineNumbers = value;

GitUI/Editor/FileViewerInternal.cs

@@ -3,6 +3,7 @@
 using System.Threading.Tasks;
 using System.Windows.Forms;
 using GitCommands;
+using GitExtUtils.GitUI;
 using GitUI.Editor.Diff;
 using ICSharpCode.TextEditor;
 using ICSharpCode.TextEditor.Document;
@@ -104,11 +105,7 @@ public string GetText()
             return TextEditor.Text;
         }
 
-        public bool ShowLineNumbers
-        {
-            get => TextEditor.ShowLineNumbers;
-            set => TextEditor.ShowLineNumbers = value;
-        }
+        public bool? ShowLineNumbers { get; set; }
 
         public void SetText(string text, Action openWithDifftool, bool isDiff = false)
         {
@@ -137,12 +134,30 @@ public void SetText(string text, Action openWithDifftool, bool isDiff = false)
 
             // important to set after the text was changed
             // otherwise the may be rendering artifacts as noted in #5568
-            TextEditor.ShowLineNumbers = !isDiff;
+            TextEditor.ShowLineNumbers = ShowLineNumbers ?? !isDiff;
+            if (ShowLineNumbers.HasValue && !ShowLineNumbers.Value)
+            {
+                Padding = new Padding(DpiUtil.Scale(5), Padding.Top, Padding.Right, Padding.Bottom);
+            }
+
             TextEditor.Refresh();
 
             _currentViewPositionCache.Restore(isDiff);
         }
 
+        private static readonly SolidBrush PaddingBrush = new SolidBrush(Color.White);
+        protected override void OnPaintBackground(PaintEventArgs e)
+        {
+            if (ShowLineNumbers.HasValue && !ShowLineNumbers.Value)
+            {
+                e.Graphics.FillRectangle(PaddingBrush, e.ClipRectangle);
+            }
+            else
+            {
+                base.OnPaintBackground(e);
+            }
+        }
+
         public void SetHighlighting(string syntax)
         {
             TextEditor.SetHighlighting(syntax);

GitUI/Editor/IFileViewer.cs

@@ -47,7 +47,7 @@ public interface IFileViewer
         Action OpenWithDifftool { get; }
         int VScrollPosition { get; set; }
 
-        bool ShowLineNumbers { get; set; }
+        bool? ShowLineNumbers { get; set; }
         bool ShowEOLMarkers { get; set; }
         bool ShowSpaces { get; set; }
         bool ShowTabs { get; set; }