AppTestHelper.AssertAnsiSnapshot + make ToAnsi platform-independent

.gitattributes

@@ -8,6 +8,10 @@
 *.sh text eol=lf
 *.ps1 text eol=lf
 
+# ANSI snapshot goldens: raw escape-sequence streams with LF row breaks.
+# Must NOT be EOL-normalized or the byte-exact compare + `cat` fidelity breaks.
+*.ans binary
+
 # Denote all files that are truly binary and should not be modified.
 *.png binary
 *.jpg binary

Terminal.Gui/Drivers/Output/OutputBase.cs

@@ -400,10 +400,14 @@ protected void BuildAnsiForRegion (IOutputBuffer buffer,
                 lastUrl = null;
             }
 
-            // Add newline at end of row if requested
+            // Add newline at end of row if requested. Use a fixed '\n', NOT
+            // StringBuilder.AppendLine () / Environment.NewLine: ToAnsi produces a portable
+            // escape-sequence stream that must be byte-identical regardless of the OS it ran
+            // on (golden snapshots, cross-platform diffing). Terminals map LF -> CRLF via the
+            // ONLCR tty discipline, so a '\n' row break still recreates the screen correctly.
             if (addNewlines)
             {
-                output.AppendLine ();
+                output.Append ('\n');
             }
         }
     }
@@ -479,7 +483,8 @@ public string ToAnsi (IOutputBuffer buffer)
                     }
                 }
 
-                output.AppendLine ();
+                // Fixed '\n' (not Environment.NewLine) — keep legacy-console output portable too.
+                output.Append ('\n');
             }
 
             return output.ToString ();

Tests/AppTestHelpers/AnsiSnapshotException.cs

@@ -0,0 +1,12 @@
+namespace AppTestHelpers;
+
+/// <summary>
+///     Thrown by <see cref="AppTestHelper.AssertAnsiSnapshot" /> when the rendered screen does
+///     not match the recorded golden. Deliberately framework-agnostic (no xunit/nunit
+///     dependency) — any test runner reports a thrown exception as a failure.
+/// </summary>
+public sealed class AnsiSnapshotException : Exception
+{
+    /// <inheritdoc cref="AnsiSnapshotException" />
+    public AnsiSnapshotException (string message) : base (message) { }
+}

Tests/AppTestHelpers/AppTestHelper.Snapshot.cs

@@ -0,0 +1,112 @@
+using System.Runtime.CompilerServices;
+using System.Text;
+
+namespace AppTestHelpers;
+
+public partial class AppTestHelper
+{
+    /// <summary>
+    ///     Asserts the current screen against a golden <b>ANSI</b> snapshot file.
+    /// </summary>
+    /// <remarks>
+    ///     <para>
+    ///         Captures the screen via <c>IDriver.ToAnsi ()</c> — the exact escape-sequence stream
+    ///         the driver would write to recreate it (truecolor, bold, reverse, blink, layout),
+    ///         excluding the terminal cursor (a separate, non-deterministic <c>SetCursor</c>, so
+    ///         snapshots stay stable). Row separators are normalized to <c>\n</c> so the same
+    ///         golden compares on every platform. The recorded <c>.ans</c> file <i>is</i> the look:
+    ///         <c>cat &lt;file&gt;.ans</c> in a truecolor terminal reproduces the screen exactly.
+    ///     </para>
+    ///     <para>
+    ///         Complements <see cref="AnsiScreenShot" /> (which only dumps to a writer): this
+    ///         records on first run (or when the <c>UPDATE_SNAPSHOTS</c> environment variable is
+    ///         <c>1</c>/<c>true</c>) and otherwise compares byte-for-byte. On mismatch it writes a
+    ///         sibling <c>.ans.actual</c> and throws with the plain-text render inline plus the
+    ///         <c>cat</c> commands — enough to verify the look without an interactive run. Set
+    ///         <c>SNAPSHOT_DIR</c> to override the golden root (default: <c>__snapshots__/</c>
+    ///         beside the calling test source).
+    ///     </para>
+    /// </remarks>
+    /// <param name="name">Snapshot name, unique within the test (becomes <c>&lt;name&gt;.ans</c>).</param>
+    /// <param name="callerFile">Compiler-supplied; locates <c>__snapshots__/</c> beside the test.</param>
+    /// <returns>This <see cref="AppTestHelper" /> (fluent).</returns>
+    public AppTestHelper AssertAnsiSnapshot (string name, [CallerFilePath] string callerFile = "")
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace (name);
+
+        string? ansi = null;
+        string? plain = null;
+
+        WaitIteration (app =>
+                       {
+                           ansi = app.Driver?.ToAnsi ();
+                           plain = app.Driver?.ToString ();
+                       });
+
+        ansi = NormalizeAnsiLineEndings (ansi ?? string.Empty);
+
+        string dir = SnapshotDirectory (callerFile);
+        Directory.CreateDirectory (dir);
+        string path = Path.Combine (dir, name + ".ans");
+
+        bool update = Environment.GetEnvironmentVariable ("UPDATE_SNAPSHOTS") is "1" or "true";
+
+        if (update || !File.Exists (path))
+        {
+            // Byte-exact, UTF-8 without BOM, no newline translation: the file must remain a
+            // faithful `cat`-able reproduction of the terminal stream. Mark *.ans `binary` in
+            // .gitattributes so core.autocrlf cannot corrupt it.
+            File.WriteAllText (path, ansi, new UTF8Encoding (false));
+
+            return this;
+        }
+
+        string expected = NormalizeAnsiLineEndings (File.ReadAllText (path));
+
+        if (string.Equals (expected, ansi, StringComparison.Ordinal))
+        {
+            return this;
+        }
+
+        string actualPath = path + ".actual";
+        File.WriteAllText (actualPath, ansi, new UTF8Encoding (false));
+
+        throw new AnsiSnapshotException (
+            $"""
+             ANSI snapshot '{name}' did not match {path}.
+
+             Plain-text render of the actual screen (glyphs only — colors/styles omitted):
+             ----------------------------------------------------------------------
+             {plain}
+             ----------------------------------------------------------------------
+
+             Exact look (with colors/styles): cat '{actualPath}'
+             Expected look:                   cat '{path}'
+
+             If this change is intended, accept it by re-running with UPDATE_SNAPSHOTS=1
+             (or copy the .actual over the .ans).
+             """);
+    }
+
+    private static string SnapshotDirectory (string callerFile)
+    {
+        string? overrideDir = Environment.GetEnvironmentVariable ("SNAPSHOT_DIR");
+
+        if (!string.IsNullOrWhiteSpace (overrideDir))
+        {
+            return overrideDir;
+        }
+
+        string? sourceDir = Path.GetDirectoryName (callerFile);
+
+        if (string.IsNullOrEmpty (sourceDir))
+        {
+            throw new InvalidOperationException (
+                "Could not resolve the snapshot directory from the caller path. Set SNAPSHOT_DIR.");
+        }
+
+        return Path.Combine (sourceDir, "__snapshots__");
+    }
+
+    private static string NormalizeAnsiLineEndings (string ansi) => ansi.Replace ("\r\n", "\n").Replace ("\r", "\n");
+}

Tests/IntegrationTests/FluentTests/AnsiSnapshotTests.cs

@@ -0,0 +1,31 @@
+using AppTestHelpers;
+using Terminal.Gui.Drivers;
+
+namespace IntegrationTests;
+
+/// <summary>
+///     Demonstrates <see cref="AppTestHelper.AssertAnsiSnapshot" />: render a screen, capture it
+///     as pure ANSI into a golden, compare byte-for-byte thereafter. The recorded
+///     <c>__snapshots__/*.ans</c> can be <c>cat</c>'d in a truecolor terminal to see the exact
+///     look without an interactive run.
+/// </summary>
+public class AnsiSnapshotTests (ITestOutputHelper outputHelper)
+{
+    private readonly TextWriter _out = new TestOutputWriter (outputHelper);
+
+    [Fact]
+    public void AssertAnsiSnapshot_Records_Then_Compares ()
+    {
+        using AppTestHelper c = With.A<Window> (20, 4, DriverRegistry.Names.ANSI, _out)
+                                    .Add (
+                                          new Label
+                                          {
+                                              X = 1,
+                                              Y = 1,
+                                              Text = "Hello, snapshot!"
+                                          })
+                                    .WaitIteration ()
+                                    .AssertAnsiSnapshot (nameof (AssertAnsiSnapshot_Records_Then_Compares))
+                                    .Stop ();
+    }
+}

Tests/IntegrationTests/FluentTests/__snapshots__/AssertAnsiSnapshot_Records_Then_Compares.ans

@@ -0,0 +1,4 @@
+┌──────────────────┐
+│                  │
+│ Hello, snapshot! │
+└──────────────────┘

Tests/UnitTestsParallelizable/Drivers/Output/OutputBaseTests.cs

@@ -17,8 +17,9 @@ public void ToAnsi_SingleCell_NoAttribute_ReturnsGraphemeAndNewline ()
         buffer.AddStr ("A");
         string ansi = output.ToAnsi (buffer);
 
-        // Assert: single grapheme plus newline (BuildAnsiForRegion appends a newline per row)
-        Assert.Contains ("A" + Environment.NewLine, ansi);
+        // Assert: single grapheme plus a fixed '\n' row break. ToAnsi is platform-independent
+        // by contract — it must NOT emit Environment.NewLine.
+        Assert.Contains ("A\n", ansi);
     }
 
     [Theory]
@@ -72,8 +73,9 @@ public void ToAnsi_WithAttribute_AppendsCorrectColorSequence_BasedOnIsLegacyCons
             Assert.DoesNotContain ('\u001b', ansi);
         }
 
-        // Grapheme and newline should always be present
-        Assert.Contains ("X" + Environment.NewLine, ansi);
+        // Grapheme and a fixed '\n' row break should always be present (ToAnsi is portable;
+        // it must NOT emit Environment.NewLine).
+        Assert.Contains ("X\n", ansi);
 
         driver.Dispose ();
     }