From 1705b669479721be75e47aa7a883122de67470d6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Kotowski?= <dev@mkotowski.dev>
Date: Sun, 15 Dec 2024 21:26:04 +0100
Subject: [PATCH] fix(compat): more accurate behavior of SGR 21
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ECMA-48 and most common terminal emulators implement SGR 21 as
double underline, which predates SGR 4 underline style extension.
This behavior is much more common than "no bold" one.

Signed-off-by: Michał Kotowski <dev@mkotowski.dev>
---
 ansi/sgr.go   |  2 +-
 ansi/style.go | 13 ++++++++-----
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/ansi/sgr.go b/ansi/sgr.go
index 1a18c98e..683a9a1b 100644
--- a/ansi/sgr.go
+++ b/ansi/sgr.go
@@ -46,7 +46,7 @@ var attrStrings = map[int]string{
 	ReverseAttr:                      "7",
 	ConcealAttr:                      "8",
 	StrikethroughAttr:                "9",
-	NoBoldAttr:                       "21",
+	DoubleUnderlineAttr:              "21",
 	NormalIntensityAttr:              "22",
 	NoItalicAttr:                     "23",
 	NoUnderlineAttr:                  "24",
diff --git a/ansi/style.go b/ansi/style.go
index 7f391156..39bff95a 100644
--- a/ansi/style.go
+++ b/ansi/style.go
@@ -127,9 +127,12 @@ func (s Style) Strikethrough() Style {
 	return append(s, strikethroughAttr)
 }
 
-// NoBold appends the no bold style attribute to the style.
-func (s Style) NoBold() Style {
-	return append(s, noBoldAttr)
+// EcmaDoubleUnderline appends the double underline style attribute to the style.
+// Differs from UnderlineStyle(DoubleUnderlineStyle) by using ECMA-compliant
+// SGR 21 instead of SGR 4 subparameter extension.
+// Some terminals may interpret this as "no bold" attribute.
+func (s Style) EcmaDoubleUnderline() Style {
+	return append(s, doubleUnderlineAttr)
 }
 
 // NormalIntensity appends the normal intensity style attribute to the style.
@@ -236,7 +239,7 @@ const (
 	ReverseAttr                      Attr = 7
 	ConcealAttr                      Attr = 8
 	StrikethroughAttr                Attr = 9
-	NoBoldAttr                       Attr = 21 // Some terminals treat this as double underline.
+	DoubleUnderlineAttr              Attr = 21 // Some terminals treat this as no bold.
 	NormalIntensityAttr              Attr = 22
 	NoItalicAttr                     Attr = 23
 	NoUnderlineAttr                  Attr = 24
@@ -298,7 +301,7 @@ const (
 	reverseAttr                      = "7"
 	concealAttr                      = "8"
 	strikethroughAttr                = "9"
-	noBoldAttr                       = "21"
+	doubleUnderlineAttr              = "21"
 	normalIntensityAttr              = "22"
 	noItalicAttr                     = "23"
 	noUnderlineAttr                  = "24"