Add underline style variants and underline color support

Support double, curly, dotted, and dashed underline styles via --color
(e.g. underline-curly) and ANSI passthrough (SGR 4:N, 58, 59) with --ansi.

Close #4633
Close #4678

Thanks to @shtse8 for the test cases.

src/tui/light.go

@@ -1323,7 +1323,18 @@ func attrCodes(attr Attr) []string {
 		codes = append(codes, "3")
 	}
 	if (attr & Underline) > 0 {
-		codes = append(codes, "4")
+		switch attr.UnderlineStyle() {
+		case UlStyleDouble:
+			codes = append(codes, "4:2")
+		case UlStyleCurly:
+			codes = append(codes, "4:3")
+		case UlStyleDotted:
+			codes = append(codes, "4:4")
+		case UlStyleDashed:
+			codes = append(codes, "4:5")
+		default:
+			codes = append(codes, "4")
+		}
 	}
 	if (attr & Blink) > 0 {
 		codes = append(codes, "5")
@@ -1361,8 +1372,27 @@ func colorCodes(fg Color, bg Color) []string {
 	return codes
 }
 
-func (w *LightWindow) csiColor(fg Color, bg Color, attr Attr) (bool, string) {
+func ulColorCode(c Color) string {
+	if c == colDefault {
+		return ""
+	}
+	if c.is24() {
+		r := (c >> 16) & 0xff
+		g := (c >> 8) & 0xff
+		b := (c) & 0xff
+		return fmt.Sprintf("58;2;%d;%d;%d", r, g, b)
+	}
+	if c >= 0 && c < 256 {
+		return fmt.Sprintf("58;5;%d", c)
+	}
+	return ""
+}
+
+func (w *LightWindow) csiColor(fg Color, bg Color, ul Color, attr Attr) (bool, string) {
 	codes := append(attrCodes(attr), colorCodes(fg, bg)...)
+	if ulCode := ulColorCode(ul); ulCode != "" {
+		codes = append(codes, ulCode)
+	}
 	code := w.csi(";" + strings.Join(codes, ";") + "m")
 	return len(codes) > 0, code
 }
@@ -1376,13 +1406,13 @@ func cleanse(str string) string {
 }
 
 func (w *LightWindow) CPrint(pair ColorPair, text string) {
-	_, code := w.csiColor(pair.Fg(), pair.Bg(), pair.Attr())
+	_, code := w.csiColor(pair.Fg(), pair.Bg(), pair.Ul(), pair.Attr())
 	w.stderrInternal(cleanse(text), false, code)
 	w.csi("0m")
 }
 
 func (w *LightWindow) cprint2(fg Color, bg Color, attr Attr, text string) {
-	hasColors, code := w.csiColor(fg, bg, attr)
+	hasColors, code := w.csiColor(fg, bg, colDefault, attr)
 	if hasColors {
 		defer w.csi("0m")
 	}
@@ -1472,7 +1502,7 @@ func (w *LightWindow) fill(str string, resetCode string) FillReturn {
 
 func (w *LightWindow) setBg() string {
 	if w.bg != colDefault {
-		_, code := w.csiColor(colDefault, w.bg, AttrRegular)
+		_, code := w.csiColor(colDefault, w.bg, colDefault, AttrRegular)
 		return code
 	}
 	// Should clear dim attribute after ␍ in the preview window
@@ -1494,7 +1524,7 @@ func (w *LightWindow) Fill(text string) FillReturn {
 	return w.fill(text, code)
 }
 
-func (w *LightWindow) CFill(fg Color, bg Color, attr Attr, text string) FillReturn {
+func (w *LightWindow) CFill(fg Color, bg Color, ul Color, attr Attr, text string) FillReturn {
 	w.Move(w.posy, w.posx)
 	if fg == colDefault {
 		fg = w.fg
@@ -1502,7 +1532,7 @@ func (w *LightWindow) CFill(fg Color, bg Color, attr Attr, text string) FillRetu
 	if bg == colDefault {
 		bg = w.bg
 	}
-	if hasColors, resetCode := w.csiColor(fg, bg, attr); hasColors {
+	if hasColors, resetCode := w.csiColor(fg, bg, ul, attr); hasColors {
 		defer w.csi("0m")
 		return w.fill(text, resetCode)
 	}

src/tui/tcell.go

@@ -825,6 +825,21 @@ func (w *TcellWindow) withUrl(style tcell.Style) tcell.Style {
 	return style
 }
 
+func underlineStyleFromAttr(a Attr) tcell.UnderlineStyle {
+	switch a.UnderlineStyle() {
+	case UlStyleDouble:
+		return tcell.UnderlineStyleDouble
+	case UlStyleCurly:
+		return tcell.UnderlineStyleCurly
+	case UlStyleDotted:
+		return tcell.UnderlineStyleDotted
+	case UlStyleDashed:
+		return tcell.UnderlineStyleDashed
+	default:
+		return tcell.UnderlineStyleSolid
+	}
+}
+
 func (w *TcellWindow) printString(text string, pair ColorPair) {
 	lx := 0
 	a := pair.Attr()
@@ -833,11 +848,18 @@ func (w *TcellWindow) printString(text string, pair ColorPair) {
 	if a&AttrClear == 0 {
 		style = style.
 			Reverse(a&Attr(tcell.AttrReverse) != 0).
-			Underline(a&Attr(tcell.AttrUnderline) != 0).
 			StrikeThrough(a&Attr(tcell.AttrStrikeThrough) != 0).
 			Italic(a&Attr(tcell.AttrItalic) != 0).
 			Blink(a&Attr(tcell.AttrBlink) != 0).
 			Dim(a&Attr(tcell.AttrDim) != 0)
+		if a&Attr(tcell.AttrUnderline) != 0 {
+			style = style.Underline(underlineStyleFromAttr(a))
+			if pair.Ul() != colDefault {
+				style = style.Underline(asTcellColor(pair.Ul()))
+			}
+		} else {
+			style = style.Underline(false)
+		}
 	}
 	style = w.withUrl(style)
 
@@ -887,9 +909,16 @@ func (w *TcellWindow) fillString(text string, pair ColorPair) FillReturn {
 		Bold(a&Attr(tcell.AttrBold) != 0 || a&BoldForce != 0).
 		Dim(a&Attr(tcell.AttrDim) != 0).
 		Reverse(a&Attr(tcell.AttrReverse) != 0).
-		Underline(a&Attr(tcell.AttrUnderline) != 0).
 		StrikeThrough(a&Attr(tcell.AttrStrikeThrough) != 0).
 		Italic(a&Attr(tcell.AttrItalic) != 0)
+	if a&Attr(tcell.AttrUnderline) != 0 {
+		style = style.Underline(underlineStyleFromAttr(a))
+		if pair.Ul() != colDefault {
+			style = style.Underline(asTcellColor(pair.Ul()))
+		}
+	} else {
+		style = style.Underline(false)
+	}
 	style = w.withUrl(style)
 
 	gr := uniseg.NewGraphemes(text)
@@ -967,14 +996,14 @@ func (w *TcellWindow) LinkEnd() {
 	w.params = nil
 }
 
-func (w *TcellWindow) CFill(fg Color, bg Color, a Attr, str string) FillReturn {
+func (w *TcellWindow) CFill(fg Color, bg Color, ul Color, a Attr, str string) FillReturn {
 	if fg == colDefault {
 		fg = w.normal.Fg()
 	}
 	if bg == colDefault {
 		bg = w.normal.Bg()
 	}
-	return w.fillString(str, NewColorPair(fg, bg, a))
+	return w.fillString(str, NewColorPair(fg, bg, a).WithUl(ul))
 }
 
 func (w *TcellWindow) DrawBorder() {

src/tui/tui.go

@@ -17,15 +17,34 @@ const (
 	BoldForce     = Attr(1 << 10)
 	FullBg        = Attr(1 << 11)
 	Strip         = Attr(1 << 12)
+
+	// Underline style stored in bits 13-15 (3 bits, values 0-4)
+	// Only meaningful when the Underline attribute bit is also set.
+	// 0 = solid (default)
+	UnderlineStyleShift = 13
+	UnderlineStyleMask  = Attr(0b111 << UnderlineStyleShift)
+	UlStyleDouble       = Attr(0b001 << UnderlineStyleShift)
+	UlStyleCurly        = Attr(0b010 << UnderlineStyleShift)
+	UlStyleDotted       = Attr(0b011 << UnderlineStyleShift)
+	UlStyleDashed       = Attr(0b100 << UnderlineStyleShift)
 )
 
+func (a Attr) UnderlineStyle() Attr {
+	return a & UnderlineStyleMask
+}
+
 func (a Attr) Merge(b Attr) Attr {
 	if b&AttrRegular > 0 {
 		// Only keep bold attribute set by the system
 		return (b &^ AttrRegular) | (a & BoldForce)
 	}
 
-	return (a &^ AttrRegular) | b
+	merged := (a &^ AttrRegular) | b
+	// When b sets Underline, use b's underline style instead of OR'ing
+	if b&Underline > 0 {
+		merged = (merged &^ UnderlineStyleMask) | (b & UnderlineStyleMask)
+	}
+	return merged
 }
 
 // Types of user action
@@ -352,6 +371,7 @@ const (
 type ColorPair struct {
 	fg   Color
 	bg   Color
+	ul   Color
 	attr Attr
 }
 
@@ -363,11 +383,11 @@ func HexToColor(rrggbb string) Color {
 }
 
 func NewColorPair(fg Color, bg Color, attr Attr) ColorPair {
-	return ColorPair{fg, bg, attr}
+	return ColorPair{fg, bg, colDefault, attr}
 }
 
 func NoColorPair() ColorPair {
-	return ColorPair{-1, -1, 0}
+	return ColorPair{-1, -1, -1, 0}
 }
 
 func (p ColorPair) Fg() Color {
@@ -378,6 +398,16 @@ func (p ColorPair) Bg() Color {
 	return p.bg
 }
 
+func (p ColorPair) Ul() Color {
+	return p.ul
+}
+
+func (p ColorPair) WithUl(ul Color) ColorPair {
+	dup := p
+	dup.ul = ul
+	return dup
+}
+
 func (p ColorPair) Attr() Attr {
 	return p.attr
 }
@@ -404,6 +434,9 @@ func (p ColorPair) merge(other ColorPair, except Color) ColorPair {
 	if other.bg != except {
 		dup.bg = other.bg
 	}
+	if other.ul != except {
+		dup.ul = other.ul
+	}
 	return dup
 }
 
@@ -415,13 +448,13 @@ func (p ColorPair) WithAttr(attr Attr) ColorPair {
 
 func (p ColorPair) WithFg(fg ColorAttr) ColorPair {
 	dup := p
-	fgPair := ColorPair{fg.Color, colUndefined, fg.Attr}
+	fgPair := ColorPair{fg.Color, colUndefined, colUndefined, fg.Attr}
 	return dup.Merge(fgPair)
 }
 
 func (p ColorPair) WithBg(bg ColorAttr) ColorPair {
 	dup := p
-	bgPair := ColorPair{colUndefined, bg.Color, bg.Attr}
+	bgPair := ColorPair{colUndefined, bg.Color, colUndefined, bg.Attr}
 	return dup.Merge(bgPair)
 }
 
@@ -783,7 +816,7 @@ type Window interface {
 	Print(text string)
 	CPrint(color ColorPair, text string)
 	Fill(text string) FillReturn
-	CFill(fg Color, bg Color, attr Attr, text string) FillReturn
+	CFill(fg Color, bg Color, ul Color, attr Attr, text string) FillReturn
 	LinkBegin(uri string, params string)
 	LinkEnd()
 	Erase()
@@ -1271,7 +1304,7 @@ func initPalette(theme *ColorTheme) {
 		if fg.Color == colDefault && (fg.Attr&Reverse) > 0 {
 			bg.Color = colDefault
 		}
-		return ColorPair{fg.Color, bg.Color, fg.Attr}
+		return ColorPair{fg.Color, bg.Color, colDefault, fg.Attr}
 	}
 	blank := theme.ListFg
 	blank.Attr = AttrRegular