Addresses #4058. Basic support for non-color text styles.

Terminal.Gui/ConsoleDrivers/EscSeqUtils/EscSeqUtils.cs

@@ -1849,6 +1849,136 @@ public static void CSI_AppendBackgroundColorRGB (StringBuilder builder, int r, i
 
     #endregion
 
+    #region Text Styles
+
+    /// <summary>
+    /// Appends an ANSI SGR (Select Graphic Rendition) escape sequence to switch printed text from one <see cref="TextStyle"/> to another.
+    /// </summary>
+    /// <param name="output"><see cref="StringBuilder"/> to add escape sequence to.</param>
+    /// <param name="prev">Previous <see cref="TextStyle"/> to change away from.</param>
+    /// <param name="next">Next <see cref="TextStyle"/> to change to.</param>
+    /// <remarks>
+    /// <para>
+    /// Unlike colors, most text styling options are not mutually exclusive with each other, and can be applied independently. This creates a problem when
+    /// switching from one style to another: For instance, if your previous style is just bold, and your next style is just italic, then simply adding the
+    /// sequence to enable italic text would cause the text to remain bold. This method automatically handles this problem, enabling and disabling styles as
+    /// necessary to apply exactly the next style.
+    /// </para>
+    /// </remarks>
+    internal static void CSI_AppendTextStyleChange (StringBuilder output, TextStyle prev, TextStyle next)
+    {
+        // Do nothing if styles are the same, as no changes are necessary.
+        if (prev == next)
+        {
+            return;
+        }
+
+        // Bitwise operations to determine flag changes. A ^ B are the flags different between two flag sets. These different flags that exist in the next flag
+        // set (diff & next) are the ones that were enabled in the switch, those that exist in the previous flag set (diff & prev) are the ones that were
+        // disabled.
+        var diff = prev ^ next;
+        var enabled = diff & next;
+        var disabled = diff & prev;
+
+        // List of escape codes to apply.
+        var sgr = new List<int> ();
+
+        if (disabled != TextStyle.None)
+        {
+            // Special case: Both bold and faint have the same disabling code. While unusual, it can be valid to have both enabled at the same time, so when
+            // one and only one of them is being disabled, we need to re-enable the other afterward. We can check what flags remain enabled by taking
+            // prev & next, as this is the set of flags both have.
+            if (disabled.HasFlag (TextStyle.Bold))
+            {
+                sgr.Add (22);
+
+                if ((prev & next).HasFlag (TextStyle.Faint))
+                {
+                    sgr.Add (2);
+                }
+            }
+
+            if (disabled.HasFlag (TextStyle.Faint))
+            {
+                sgr.Add (22);
+
+                if ((prev & next).HasFlag (TextStyle.Bold))
+                {
+                    sgr.Add (1);
+                }
+            }
+
+            if (disabled.HasFlag (TextStyle.Italic))
+            {
+                sgr.Add (23);
+            }
+
+            if (disabled.HasFlag (TextStyle.Underline))
+            {
+                sgr.Add (24);
+            }
+
+            if (disabled.HasFlag (TextStyle.Blink))
+            {
+                sgr.Add (25);
+            }
+
+            if (disabled.HasFlag (TextStyle.Reverse))
+            {
+                sgr.Add (27);
+            }
+
+            if (disabled.HasFlag (TextStyle.Strikethrough))
+            {
+                sgr.Add (29);
+            }
+        }
+
+        if (enabled != TextStyle.None)
+        {
+            if (enabled.HasFlag (TextStyle.Bold))
+            {
+                sgr.Add (1);
+            }
+
+            if (enabled.HasFlag (TextStyle.Faint))
+            {
+                sgr.Add (2);
+            }
+
+            if (enabled.HasFlag (TextStyle.Italic))
+            {
+                sgr.Add (3);
+            }
+
+            if (enabled.HasFlag (TextStyle.Underline))
+            {
+                sgr.Add (4);
+            }
+
+            if (enabled.HasFlag (TextStyle.Blink))
+            {
+                sgr.Add (5);
+            }
+
+            if (enabled.HasFlag (TextStyle.Reverse))
+            {
+                sgr.Add (7);
+            }
+
+            if (enabled.HasFlag (TextStyle.Strikethrough))
+            {
+                sgr.Add (9);
+            }
+        }
+
+        output.Append ("\x1b[");
+        output.Append (string.Join (';', sgr));
+        output.Append ('m');
+    }
+
+    #endregion Text Styles
+
     #region Requests
 
     /// <summary>

Terminal.Gui/ConsoleDrivers/V2/NetOutput.cs

@@ -12,6 +12,9 @@ public class NetOutput : IConsoleOutput
 
     private CursorVisibility? _cachedCursorVisibility;
 
+    // Last text style used, for updating style with EscSeqUtils.CSI_AppendTextStyleChange().
+    private TextStyle _redrawTextStyle = TextStyle.None;
+
     /// <summary>
     ///     Creates a new instance of the <see cref="NetOutput"/> class.
     /// </summary>
@@ -134,6 +137,10 @@ public void Write (IOutputBuffer buffer)
                             attr.Background.G,
                             attr.Background.B
                         );
+
+                        EscSeqUtils.CSI_AppendTextStyleChange (output, _redrawTextStyle, attr.TextStyle);
+
+                        _redrawTextStyle = attr.TextStyle;
                     }
 
                     outputWidth++;

Terminal.Gui/ConsoleDrivers/V2/OutputBuffer.cs

@@ -33,7 +33,8 @@ public Attribute CurrentAttribute
             // TODO: This makes IConsoleDriver dependent on Application, which is not ideal. Once Attribute.PlatformColor is removed, this can be fixed.
             if (Application.Driver is { })
             {
-                _currentAttribute = new (value.Foreground, value.Background);
+                // TODO: Update this when attributes can include TextStyle in the constructor
+                _currentAttribute = new (value.Foreground, value.Background) { TextStyle = value.TextStyle };
 
                 return;
             }

Terminal.Gui/ConsoleDrivers/V2/WindowsOutput.cs

@@ -61,6 +61,9 @@ private enum DesiredAccess : uint
 
     private readonly nint _screenBuffer;
 
+    // Last text style used, for updating style with EscSeqUtils.CSI_AppendTextStyleChange().
+    private TextStyle _redrawTextStyle = TextStyle.None;
+
     public WindowsOutput ()
     {
         Logging.Logger.LogInformation ($"Creating {nameof (WindowsOutput)}");
@@ -233,6 +236,8 @@ public bool WriteToConsole (Size size, ExtendedCharInfo [] charInfoBuffer, Coord
                     prev = attr;
                     EscSeqUtils.CSI_AppendForegroundColorRGB (stringBuilder, attr.Foreground.R, attr.Foreground.G, attr.Foreground.B);
                     EscSeqUtils.CSI_AppendBackgroundColorRGB (stringBuilder, attr.Background.R, attr.Background.G, attr.Background.B);
+                    EscSeqUtils.CSI_AppendTextStyleChange (stringBuilder, _redrawTextStyle, attr.TextStyle);
+                    _redrawTextStyle = attr.TextStyle;
                 }
 
                 if (info.Char != '\x1b')

Terminal.Gui/Drawing/Attribute.cs

@@ -34,6 +34,10 @@ namespace Terminal.Gui;
     [JsonConverter (typeof (ColorJsonConverter))]
     public Color Background { get; }
 
+    // TODO: Add constructors which permit including a TextStyle.
+    /// <summary>The text style (bold, italic, underlined, etc.).</summary>
+    public TextStyle TextStyle { get; init; } = TextStyle.None;
+
     /// <summary>Initializes a new instance with default values.</summary>
     public Attribute ()
     {
@@ -103,6 +107,7 @@ public Attribute (in Color color) : this (color, color) { }
     /// <inheritdoc/>
     public override int GetHashCode () { return HashCode.Combine (PlatformColor, Foreground, Background); }
 
+    // TODO: Add TextStyle to Attribute.ToString(), modify unit tests to account
     /// <inheritdoc/>
     public override string ToString ()
     {

Terminal.Gui/Drawing/TextStyle.cs

@@ -0,0 +1,79 @@
+namespace Terminal.Gui;
+
+/// <summary>
+///     Defines non-color text style flags for an <see cref="Attribute"/>.
+/// </summary>
+/// <remarks>
+///     <para>
+///         Only a subset of ANSI SGR (Select Graphic Rendition) styles are represented.
+///         Styles that are poorly supported, non-visual, or redundant with other APIs are omitted.
+///     </para>
+///     <para>
+///         Multiple styles can be combined using bitwise operations. Use <see cref="Attribute.TextStyle"/>
+///         to get or set these styles on an <see cref="Attribute"/>.
+///     </para>
+///     <para>
+///         Note that <see cref="TextStyle.Bold"/> and <see cref="TextStyle.Faint"/> may be mutually exclusive depending on
+///         the user's terminal and its settings. For instance, if a terminal displays faint text as a darker color, and
+///         bold text as a lighter color, then both cannot
+///         be shown at the same time, and it will be up to the terminal to decide which to display.
+///     </para>
+/// </remarks>
+[Flags]
+public enum TextStyle : byte
+{
+    /// <summary>
+    ///     No text style.
+    /// </summary>
+    /// <remarks>Corresponds to no active SGR styles.</remarks>
+    None = 0b_0000_0000,
+
+    /// <summary>
+    ///     Bold text.
+    /// </summary>
+    /// <remarks>
+    ///     SGR code: 1 (Bold). May be mutually exclusive with <see cref="TextStyle.Faint"/>, see <see cref="TextStyle"/>
+    ///     remarks.
+    /// </remarks>
+    Bold = 0b_0000_0001,
+
+    /// <summary>
+    ///     Faint (dim) text.
+    /// </summary>
+    /// <remarks>
+    ///     SGR code: 2 (Faint). Not widely supported on all terminals. May be mutually exclusive with
+    ///     <see cref="TextStyle.Bold"/>, see
+    ///     <see cref="TextStyle"/> remarks.
+    /// </remarks>
+    Faint = 0b_0000_0010,
+
+    /// <summary>
+    ///     Italic text.
+    /// </summary>
+    /// <remarks>SGR code: 3 (Italic). Some terminals may not support italic rendering.</remarks>
+    Italic = 0b_0000_0100,
+
+    /// <summary>
+    ///     Underlined text.
+    /// </summary>
+    /// <remarks>SGR code: 4 (Underline).</remarks>
+    Underline = 0b_0000_1000,
+
+    /// <summary>
+    ///     Slow blinking text.
+    /// </summary>
+    /// <remarks>SGR code: 5 (Slow Blink). Support varies; blinking is often disabled in modern terminals.</remarks>
+    Blink = 0b_0001_0000,
+
+    /// <summary>
+    ///     Reverse video (swaps foreground and background colors).
+    /// </summary>
+    /// <remarks>SGR code: 7 (Reverse Video).</remarks>
+    Reverse = 0b_0010_0000,
+
+    /// <summary>
+    ///     Strikethrough (crossed-out) text.
+    /// </summary>
+    /// <remarks>SGR code: 9 (Crossed-out / Strikethrough).</remarks>
+    Strikethrough = 0b_0100_0000
+}