Add UseThemeBackground property to Editor

examples/ted/TedApp.cs

@@ -74,6 +74,14 @@ public TedApp (bool readOnly = false)
             Editor.ConvertTabsToSpaces = e.NewValue == CheckState.Checked;
         };
 
+        CheckBox useThemeBackgroundCheckBox = new ()
+        {
+            AllowCheckStateNone = false,
+            CanFocus = false,
+            Text = "Use _Theme Background",
+            Value = Editor.UseThemeBackground ? CheckState.Checked : CheckState.UnChecked
+        };
+
         ThemeDropDown = new DropDownList<ThemeName>
         {
             Value = ThemeName.DarkPlus,
@@ -191,6 +199,15 @@ public TedApp (bool readOnly = false)
                 {
                     CommandView = convertTabsToSpacesCheckBox,
                     HelpText = "Insert spaces when Tab is pressed"
+                },
+                new MenuItem
+                {
+                    Action = () =>
+                    {
+                        Editor.UseThemeBackground = useThemeBackgroundCheckBox.Value == CheckState.Checked;
+                    },
+                    CommandView = useThemeBackgroundCheckBox,
+                    HelpText = "Use theme background for highlighted text"
                 }
             ]),
             new MenuBarItem (Strings.menuHelp,

src/Terminal.Gui.Editor/Editor.Drawing.cs

@@ -22,13 +22,46 @@ protected override bool OnDrawingContent (DrawContext? context)
         Attribute normal = GetAttributeForRole (VisualRole.Normal);
         Attribute selected = GetAttributeForRole (VisualRole.Active);
 
+        FillViewportBackground (viewport, normal);
         DrawVisibleLines (viewport, normal, selected);
         SetAttribute (normal);
         UpdateCursor ();
 
         return true;
     }
 
+    /// <summary>
+    ///     When <see cref="UseThemeBackground" /> is <see langword="true" /> and a syntax highlighter
+    ///     provides a <see cref="ISyntaxHighlighter.DefaultBackground" />, fills the viewport with
+    ///     that background so empty cells match per-token backgrounds.
+    /// </summary>
+    private void FillViewportBackground (Rectangle viewport, Attribute normal)
+    {
+        if (!UseThemeBackground)
+        {
+            return;
+        }
+
+#pragma warning disable CS0618 // Type or member is obsolete
+        Terminal.Gui.Drawing.Color? themeBg = SyntaxHighlighter?.DefaultBackground;
+#pragma warning restore CS0618 // Type or member is obsolete
+
+        if (themeBg is not { } bg)
+        {
+            return;
+        }
+
+        Attribute fillAttr = new (normal.Foreground, bg);
+        SetAttribute (fillAttr);
+
+        var spaces = new string (' ', viewport.Width);
+
+        for (var row = 0; row < viewport.Height; row++)
+        {
+            AddStr (0, row, spaces);
+        }
+    }
+
     private void DrawVisibleLines (Rectangle viewport, Attribute normal, Attribute selected)
     {
         // The CS0618 here is the API's purpose: SyntaxHighlighter is [Obsolete] to warn

src/Terminal.Gui.Editor/Editor.cs

@@ -231,6 +231,29 @@ public int IndentationSize
     /// <summary>Whether pressing Tab inserts spaces instead of a tab character.</summary>
     public bool ConvertTabsToSpaces { get; set; }
 
+    /// <summary>
+    ///     When <see langword="true" /> (the default), syntax-highlighted tokens keep both their
+    ///     foreground and background from the syntax highlighting theme (e.g. Dark Plus's
+    ///     <c>#1E1E1E</c> background). Set to <see langword="false" /> to override the
+    ///     highlighter's background with the TG scheme's <see cref="VisualRole.Normal" />
+    ///     background, matching the active application theme.
+    /// </summary>
+    public bool UseThemeBackground
+    {
+        get;
+        set
+        {
+            if (field == value)
+            {
+                return;
+            }
+
+            field = value;
+            ClearVisualLineCaches ();
+            SetNeedsDraw ();
+        }
+    } = true;
+
     /// <summary>Whether tab characters render with a visible glyph in their first cell.</summary>
     public bool ShowTabs
     {
@@ -731,7 +754,8 @@ private CellVisualLine BuildVisualLine (
             styledSegments,
             selectionStart,
             selectionEnd,
-            LineTransformers);
+            LineTransformers,
+            UseThemeBackground);
 
         return _visualLineBuilder.Build (line, context);
     }

src/Terminal.Gui.Editor/Rendering/VisualLineBuildContext.cs

@@ -14,7 +14,8 @@ public sealed class VisualLineBuildContext (
     IReadOnlyList<StyledSegment>? styledSegments,
     int selectionStart,
     int selectionEnd,
-    IEnumerable<IVisualLineTransformer> lineTransformers)
+    IEnumerable<IVisualLineTransformer> lineTransformers,
+    bool useThemeBackground = true)
 {
     public TextDocument Document { get; } = document;
 
@@ -34,5 +35,12 @@ public sealed class VisualLineBuildContext (
 
     public IEnumerable<IVisualLineTransformer> LineTransformers { get; } = lineTransformers;
 
+    /// <summary>
+    ///     When <see langword="true" />, styled-segment backgrounds are preserved from the
+    ///     syntax highlighting theme. When <see langword="false" />, backgrounds are replaced
+    ///     with <see cref="NormalAttribute" />'s background so text blends into the TG scheme.
+    /// </summary>
+    public bool UseThemeBackground { get; } = useThemeBackground;
+
     public bool HasSelection => SelectionStart < SelectionEnd;
 }

src/Terminal.Gui.Editor/Rendering/VisualLineBuilder.cs

@@ -157,8 +157,15 @@ private static Attribute GetAttribute (VisualLineBuildContext context, int segme
             return context.NormalAttribute;
         }
 
-        return context.StyledSegments[Math.Min (segmentIndex, context.StyledSegments.Count - 1)].Attribute
-               ?? context.NormalAttribute;
+        Attribute segmentAttribute = context.StyledSegments[Math.Min (segmentIndex, context.StyledSegments.Count - 1)].Attribute
+                                     ?? context.NormalAttribute;
+
+        if (!context.UseThemeBackground)
+        {
+            return new Attribute (segmentAttribute.Foreground, context.NormalAttribute.Background);
+        }
+
+        return segmentAttribute;
     }
 
     private static int GetSegmentEnd (IReadOnlyList<StyledSegment>? segments, int segmentIndex)

tests/Terminal.Gui.Editor.IntegrationTests/EditorRenderingTests.cs

@@ -328,4 +328,57 @@ public async Task Cursor_Position_After_Tab_Uses_Expanded_Tab_Columns ()
 
         Assert.Equal (new Point (4, 0), fx.Top.Editor.Cursor.Position);
     }
+
+    [Fact]
+    public async Task UseThemeBackground_True_Preserves_Highlighter_Background ()
+    {
+        const string text = "public class C";
+
+        await using AppFixture<EditorTestHost> fx = new (() => new EditorTestHost (text));
+#pragma warning disable CS0618 // Type or member is obsolete
+        fx.Top.Editor.SyntaxHighlighter = new TextMateSyntaxHighlighter ();
+#pragma warning restore CS0618 // Type or member is obsolete
+        fx.Top.Editor.UseThemeBackground = true;
+        fx.Render ();
+
+        TextMateSyntaxHighlighter highlighter = new ();
+        Attribute expected = highlighter.Highlight (text, "csharp")[0].Attribute!.Value;
+
+        Cell cell = fx.Driver.Contents![0, 0];
+        Assert.Equal ("p", cell.Grapheme);
+        Assert.Equal (expected, cell.Attribute);
+    }
+
+    [Fact]
+    public async Task UseThemeBackground_False_Overrides_Highlighter_Background ()
+    {
+        const string text = "public class C";
+
+        await using AppFixture<EditorTestHost> fx = new (() => new EditorTestHost (text));
+#pragma warning disable CS0618 // Type or member is obsolete
+        fx.Top.Editor.SyntaxHighlighter = new TextMateSyntaxHighlighter ();
+#pragma warning restore CS0618 // Type or member is obsolete
+        fx.Top.Editor.UseThemeBackground = false;
+        fx.Render ();
+
+        Attribute normal = fx.Top.Editor.GetAttributeForRole (VisualRole.Normal);
+        Cell cell = fx.Driver.Contents![0, 0];
+        Assert.Equal ("p", cell.Grapheme);
+
+        // The foreground should come from the highlighter (different from Normal's foreground).
+        TextMateSyntaxHighlighter highlighter = new ();
+        Attribute highlighterAttr = highlighter.Highlight (text, "csharp")[0].Attribute!.Value;
+        Assert.Equal (highlighterAttr.Foreground, cell.Attribute!.Value.Foreground);
+
+        // The background must match the TG theme's Normal background, not the highlighter's.
+        Assert.Equal (normal.Background, cell.Attribute!.Value.Background);
+    }
+
+    [Fact]
+    public async Task UseThemeBackground_Defaults_To_True ()
+    {
+        await using AppFixture<EditorTestHost> fx = new (() => new EditorTestHost ("Hello"));
+
+        Assert.True (fx.Top.Editor.UseThemeBackground);
+    }
 }