terminal: track underline color on cell

src/terminal/Screen.zig

@@ -182,6 +182,12 @@ pub const Cell = struct {
     fg: color.RGB = .{},
     bg: color.RGB = .{},
 
+    /// Underline color.
+    /// NOTE(mitchellh): This is very rarely set so ideally we wouldn't waste
+    /// cell space for this. For now its on this struct because it is convenient
+    /// but we should consider a lookaside table for this.
+    underline_fg: color.RGB = .{},
+
     /// On/off attributes that can be set
     attrs: packed struct {
         has_bg: bool = false,
@@ -194,6 +200,7 @@ pub const Cell = struct {
         inverse: bool = false,
         strikethrough: bool = false,
         underline: sgr.Attribute.Underline = .none,
+        underline_color: bool = false,
 
         /// True if this is a wide character. This char takes up
         /// two cells. The following cell ALWAYS is a space.
@@ -265,7 +272,7 @@ pub const Cell = struct {
 
     test {
         //log.warn("CELL={} bits={} {}", .{ @sizeOf(Cell), @bitSizeOf(Cell), @alignOf(Cell) });
-        try std.testing.expectEqual(12, @sizeOf(Cell));
+        try std.testing.expectEqual(16, @sizeOf(Cell));
     }
 };
 

src/terminal/Terminal.zig

@@ -461,8 +461,18 @@ pub fn setAttribute(self: *Terminal, attr: sgr.Attribute) !void {
             self.screen.cursor.pen.attrs.underline = .none;
         },
 
-        // TODO
+        .underline_color => |rgb| {
-        .underline_color, .reset_underline_color => {},
+            self.screen.cursor.pen.attrs.underline_color = true;
+            self.screen.cursor.pen.underline_fg = .{
+                .r = rgb.r,
+                .g = rgb.g,
+                .b = rgb.b,
+            };
+        },
+
+        .reset_underline_color => {
+            self.screen.cursor.pen.attrs.underline_color = false;
+        },
 
         .blink => {
             log.warn("blink requested, but not implemented", .{});