From d209fe9b1ef5cf135f86058942e3caa6f998038c Mon Sep 17 00:00:00 2001
From: nicm <nicm>
Date: Sat, 2 Sep 2023 09:17:23 +0000
Subject: [PATCH] Setulc only does RGB colour so add Setulc1 to do non-RGB
 colours, GitHub issue 3627.

---
 tmux.1         | 10 ++++++++--
 tmux.h         |  1 +
 tty-features.c |  1 +
 tty-term.c     |  1 +
 tty.c          | 24 +++++++++++++++++-------
 5 files changed, 28 insertions(+), 9 deletions(-)

diff --git a/tmux.1 b/tmux.1
index 6306e12a..abe88ba3 100644
--- a/tmux.1
+++ b/tmux.1
@@ -6802,9 +6802,15 @@ Set a styled underscore.
 The single parameter is one of: 0 for no underscore, 1 for normal
 underscore, 2 for double underscore, 3 for curly underscore, 4 for dotted
 underscore and 5 for dashed underscore.
-.It Em \&Setulc , \&ol
+.It Em \&Setulc , \&Setulc1, \&ol
 Set the underscore colour or reset to the default.
-The argument is (red * 65536) + (green * 256) + blue where each is between 0
+.Em Setulc
+is for RGB colours and
+.Em Setulc1
+for ANSI or 256 colours.
+The
+.Em Setulc
+argument is (red * 65536) + (green * 256) + blue where each is between 0
 and 255.
 .It Em \&Ss , Se
 Set or reset the cursor style.
diff --git a/tmux.h b/tmux.h
index 758713e5..4ce4d1dd 100644
--- a/tmux.h
+++ b/tmux.h
@@ -536,6 +536,7 @@ enum tty_code_code {
 	TTYC_SETRGBB,
 	TTYC_SETRGBF,
 	TTYC_SETULC,
+	TTYC_SETULC1,
 	TTYC_SGR0,
 	TTYC_SITM,
 	TTYC_SMACS,
diff --git a/tty-features.c b/tty-features.c
index 3f4fdbfd..e3293872 100644
--- a/tty-features.c
+++ b/tty-features.c
@@ -145,6 +145,7 @@ static const struct tty_feature tty_feature_overline = {
 static const char *const tty_feature_usstyle_capabilities[] = {
 	"Smulx=\\E[4::%p1%dm",
 	"Setulc=\\E[58::2::%p1%{65536}%/%d::%p1%{256}%/%{255}%&%d::%p1%{255}%&%d%;m",
+	"Setulc1=\\E[58::5::%p1%dm",
 	"ol=\\E[59m",
 	NULL
 };
diff --git a/tty-term.c b/tty-term.c
index a2798347..e26b765e 100644
--- a/tty-term.c
+++ b/tty-term.c
@@ -264,6 +264,7 @@ static const struct tty_term_code_entry tty_term_codes[] = {
 	[TTYC_SETRGBB] = { TTYCODE_STRING, "setrgbb" },
 	[TTYC_SETRGBF] = { TTYCODE_STRING, "setrgbf" },
 	[TTYC_SETULC] = { TTYCODE_STRING, "Setulc" },
+	[TTYC_SETULC1] = { TTYCODE_STRING, "Setulc1" },
 	[TTYC_SE] = { TTYCODE_STRING, "Se" },
 	[TTYC_SXL] =  { TTYCODE_FLAG, "Sxl" },
 	[TTYC_SGR0] = { TTYCODE_STRING, "sgr0" },
diff --git a/tty.c b/tty.c
index ad4a110a..4bcc68d9 100644
--- a/tty.c
+++ b/tty.c
@@ -2814,11 +2814,13 @@ tty_check_us(__unused struct tty *tty, struct colour_palette *palette,
 			gc->us = c;
 	}
 
-	/* Underscore colour is set as RGB so convert. */
-	if ((c = colour_force_rgb (gc->us)) == -1)
-		gc->us = 8;
-	else
-		gc->us = c;
+	/* Convert underscore colour if only RGB can be supported. */
+	if (!tty_term_has(tty->term, TTYC_SETULC1)) {
+		    if ((c = colour_force_rgb (gc->us)) == -1)
+			    gc->us = 8;
+		    else
+			    gc->us = c;
+	}
 }
 
 static void
@@ -2898,9 +2900,17 @@ tty_colours_us(struct tty *tty, const struct grid_cell *gc)
 		goto save;
 	}
 
-	/* Must be an RGB colour - this should never happen. */
-	if (~gc->us & COLOUR_FLAG_RGB)
+	/*
+	 * If this is not an RGB colour, use Setulc1 if it exists, otherwise
+	 * convert.
+	 */
+	if (~gc->us & COLOUR_FLAG_RGB) {
+		c = gc->us;
+		if ((~c & COLOUR_FLAG_256) && (c >= 90 && c <= 97))
+			c -= 82;
+		tty_putcode_i(tty, TTYC_SETULC1, c & ~COLOUR_FLAG_256);
 		return;
+	}
 
 	/*
 	 * Setulc and setal follows the ncurses(3) one argument "direct colour"