feat: lift IIndentationStrategy from AvaloniaEdit and wire auto-indent on Enter

Lift IIndentationStrategy and DefaultIndentationStrategy from AvaloniaEdit
(commit d7a6b63) into src/Terminal.Gui.Editor/Indentation/ with namespace
Terminal.Gui.Text.Indentation. The default strategy copies leading whitespace
from the previous line when Enter is pressed.

- Add Editor.IndentationStrategy property (defaults to DefaultIndentationStrategy)
- Wire Command.NewLine to call IndentLine on the new line after insertion
- Add Auto Indent toggle to ted's Options menu (first-class consumer)
- Add unit tests for DefaultIndentationStrategy and Editor auto-indent integration
- Update UPSTREAM.md with Indentation/ lift and modification log
- Skip CSharp/ subfolder (language-specific strategies are out of scope)

Closes #TBD

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tig

examples/ted/TedApp.FileOperations.cs

@@ -151,7 +151,7 @@ private SaveChangesChoice ShowDefaultSaveChangesDialog ()
             "Save changes?",
             "The document has unsaved changes. Save before quitting?",
             Strings.btnCancel,
-            "Don't Save",
+            "Do_n't Save",
             Strings.btnSave);
 
         return result switch

examples/ted/TedApp.cs

@@ -73,6 +73,21 @@ public TedApp (bool readOnly = false)
             Editor.ConvertTabsToSpaces = e.NewValue == CheckState.Checked;
         };
 
+        CheckBox autoIndentCheckBox = new ()
+        {
+            AllowCheckStateNone = false,
+            CanFocus = false,
+            Text = "_Auto Indent",
+            Value = Editor.IndentationStrategy is not null ? CheckState.Checked : CheckState.UnChecked
+        };
+
+        autoIndentCheckBox.ValueChanged += (_, e) =>
+        {
+            Editor.IndentationStrategy = e.NewValue == CheckState.Checked
+                ? new Terminal.Gui.Text.Indentation.DefaultIndentationStrategy ()
+                : null;
+        };
+
         CheckBox useThemeBackgroundCheckBox = new ()
         {
             AllowCheckStateNone = false,
@@ -200,6 +215,11 @@ public TedApp (bool readOnly = false)
                     HelpText = "Insert spaces when Tab is pressed"
                 },
                 new MenuItem
+                {
+                    CommandView = autoIndentCheckBox,
+                    HelpText = "Copy indentation from the previous line on Enter"
+                },
+                new MenuItem
                 {
                     Action = () =>
                     {

src/Terminal.Gui.Editor/Editor.Commands.cs

@@ -81,7 +81,7 @@ private void CreateCommandsAndBindings ()
         });
 
         // Editing — selection-aware
-        AddCommand (Command.NewLine, () => InsertOrReplace ("\n"));
+        AddCommand (Command.NewLine, InsertNewLineWithAutoIndent);
         AddCommand (Command.DeleteCharLeft, DeleteLeft);
         AddCommand (Command.DeleteCharRight, DeleteRight);
 
@@ -170,6 +170,33 @@ private void CreateCommandsAndBindings ()
         return true;
     }
 
+    private bool? InsertNewLineWithAutoIndent ()
+    {
+        if (ReadOnly)
+        {
+            return true;
+        }
+
+        if (HasSelection)
+        {
+            ReplaceSelection ("\n");
+        }
+        else
+        {
+            _document!.Insert (CaretOffset, "\n");
+        }
+
+        // After the newline is inserted the caret sits at the start of the new line.
+        // Ask the indentation strategy to fill in leading whitespace.
+        if (IndentationStrategy is { } strategy && _document is not null)
+        {
+            DocumentLine newLine = _document.GetLineByOffset (CaretOffset);
+            strategy.IndentLine (_document, newLine);
+        }
+
+        return true;
+    }
+
     private bool? InsertOrReplace (string text)
     {
         if (ReadOnly)

src/Terminal.Gui.Editor/Editor.cs

@@ -2,6 +2,7 @@
 using System.Drawing;
 using Terminal.Gui.Document;
 using Terminal.Gui.Drawing;
+using Terminal.Gui.Text.Indentation;
 using Terminal.Gui.ViewBase;
 using Terminal.Gui.Views.Rendering;
 using Attribute = Terminal.Gui.Drawing.Attribute;
@@ -231,6 +232,15 @@ public int IndentationSize
     /// <summary>Whether pressing Tab inserts spaces instead of a tab character.</summary>
     public bool ConvertTabsToSpaces { get; set; }
 
+    /// <summary>
+    ///     Gets or sets the indentation strategy applied when Enter is pressed.
+    ///     When non-null, the strategy's <see cref="IIndentationStrategy.IndentLine" /> is called
+    ///     on the newly created line to copy (or compute) indentation from the previous line.
+    ///     Defaults to <see cref="DefaultIndentationStrategy" />.
+    ///     Set to <see langword="null" /> to disable auto-indent on Enter.
+    /// </summary>
+    public IIndentationStrategy? IndentationStrategy { get; set; } = new DefaultIndentationStrategy ();
+
     /// <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

src/Terminal.Gui.Editor/Indentation/DefaultIndentationStrategy.cs

@@ -0,0 +1,58 @@
+// Adapted for Terminal.Gui from AvaloniaEdit d7a6b63
+// Copyright (c) 2014 AlphaSierraPapa for the SharpDevelop Team
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using Terminal.Gui.Document;
+
+namespace Terminal.Gui.Text.Indentation;
+
+/// <summary>
+///     Handles indentation by copying the indentation from the previous line.
+///     Does not support indenting multiple lines.
+/// </summary>
+public class DefaultIndentationStrategy : IIndentationStrategy
+{
+    /// <inheritdoc />
+    public virtual void IndentLine (TextDocument document, DocumentLine line)
+    {
+        ArgumentNullException.ThrowIfNull (document);
+        ArgumentNullException.ThrowIfNull (line);
+
+        DocumentLine? previousLine = line.PreviousLine;
+
+        if (previousLine is null)
+        {
+            return;
+        }
+
+        ISegment indentationSegment = TextUtilities.GetWhitespaceAfter (document, previousLine.Offset);
+        var indentation = document.GetText (indentationSegment);
+
+        // Copy indentation to line
+        indentationSegment = TextUtilities.GetWhitespaceAfter (document, line.Offset);
+
+        document.Replace (indentationSegment.Offset, indentationSegment.Length, indentation,
+            OffsetChangeMappingType.RemoveAndInsert);
+        // OffsetChangeMappingType.RemoveAndInsert guarantees the caret moves behind the new indentation.
+    }
+
+    /// <summary>
+    ///     Does nothing: indenting multiple lines is useless without a smart indentation strategy.
+    /// </summary>
+    public virtual void IndentLines (TextDocument document, int beginLine, int endLine) { }
+}

src/Terminal.Gui.Editor/Indentation/IIndentationStrategy.cs

@@ -0,0 +1,39 @@
+// Adapted for Terminal.Gui from AvaloniaEdit d7a6b63
+// Copyright (c) 2014 AlphaSierraPapa for the SharpDevelop Team
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using Terminal.Gui.Document;
+
+namespace Terminal.Gui.Text.Indentation;
+
+/// <summary>
+///     Strategy how the text editor handles indentation when new lines are inserted.
+/// </summary>
+public interface IIndentationStrategy
+{
+    /// <summary>
+    ///     Sets the indentation for the specified line.
+    ///     Usually this is constructed from the indentation of the previous line.
+    /// </summary>
+    void IndentLine (TextDocument document, DocumentLine line);
+
+    /// <summary>
+    ///     Reindents a set of lines.
+    /// </summary>
+    void IndentLines (TextDocument document, int beginLine, int endLine);
+}

tests/Terminal.Gui.Editor.Tests/Indentation/DefaultIndentationStrategyTests.cs

@@ -0,0 +1,115 @@
+// CoPilot - claude-opus-4.6
+
+using Terminal.Gui.Document;
+using Terminal.Gui.Text.Indentation;
+using Xunit;
+
+namespace Terminal.Gui.Editor.Tests.Indentation;
+
+/// <summary>
+///     Tests for <see cref="DefaultIndentationStrategy" /> — the AvaloniaEdit-derived strategy
+///     that copies leading whitespace from the previous line.
+/// </summary>
+public class DefaultIndentationStrategyTests
+{
+    [Fact]
+    public void IndentLine_Copies_Spaces_From_Previous_Line ()
+    {
+        TextDocument doc = new ("    hello\n");
+        DefaultIndentationStrategy strategy = new ();
+
+        DocumentLine line2 = doc.GetLineByNumber (2);
+        strategy.IndentLine (doc, line2);
+
+        Assert.Equal ("    hello\n    ", doc.Text);
+    }
+
+    [Fact]
+    public void IndentLine_Copies_Tab_From_Previous_Line ()
+    {
+        TextDocument doc = new ("\thello\n");
+        DefaultIndentationStrategy strategy = new ();
+
+        DocumentLine line2 = doc.GetLineByNumber (2);
+        strategy.IndentLine (doc, line2);
+
+        Assert.Equal ("\thello\n\t", doc.Text);
+    }
+
+    [Fact]
+    public void IndentLine_Copies_Mixed_Whitespace_From_Previous_Line ()
+    {
+        TextDocument doc = new ("\t  hello\n");
+        DefaultIndentationStrategy strategy = new ();
+
+        DocumentLine line2 = doc.GetLineByNumber (2);
+        strategy.IndentLine (doc, line2);
+
+        Assert.Equal ("\t  hello\n\t  ", doc.Text);
+    }
+
+    [Fact]
+    public void IndentLine_NoOp_On_First_Line ()
+    {
+        TextDocument doc = new ("hello");
+        DefaultIndentationStrategy strategy = new ();
+
+        DocumentLine line1 = doc.GetLineByNumber (1);
+        strategy.IndentLine (doc, line1);
+
+        Assert.Equal ("hello", doc.Text);
+    }
+
+    [Fact]
+    public void IndentLine_NoOp_When_Previous_Line_Has_No_Indentation ()
+    {
+        TextDocument doc = new ("hello\n");
+        DefaultIndentationStrategy strategy = new ();
+
+        DocumentLine line2 = doc.GetLineByNumber (2);
+        strategy.IndentLine (doc, line2);
+
+        Assert.Equal ("hello\n", doc.Text);
+    }
+
+    [Fact]
+    public void IndentLine_Replaces_Existing_Whitespace_On_Target_Line ()
+    {
+        TextDocument doc = new ("    hello\n  world");
+        DefaultIndentationStrategy strategy = new ();
+
+        DocumentLine line2 = doc.GetLineByNumber (2);
+        strategy.IndentLine (doc, line2);
+
+        Assert.Equal ("    hello\n    world", doc.Text);
+    }
+
+    [Fact]
+    public void IndentLines_Does_Nothing ()
+    {
+        TextDocument doc = new ("hello\nworld\n");
+        DefaultIndentationStrategy strategy = new ();
+
+        // IndentLines is a no-op for DefaultIndentationStrategy
+        strategy.IndentLines (doc, 1, 2);
+
+        Assert.Equal ("hello\nworld\n", doc.Text);
+    }
+
+    [Fact]
+    public void IndentLine_Throws_On_Null_Document ()
+    {
+        DefaultIndentationStrategy strategy = new ();
+
+        Assert.Throws<ArgumentNullException> (() => strategy.IndentLine (null!, null!));
+    }
+
+    [Fact]
+    public void IndentLine_Throws_On_Null_Line ()
+    {
+        TextDocument doc = new ("hello");
+        DefaultIndentationStrategy strategy = new ();
+
+        Assert.Throws<ArgumentNullException> (() => strategy.IndentLine (doc, null!));
+    }
+}