From 8b44703b41d88ea626b91c3c42e065ae207be08f Mon Sep 17 00:00:00 2001
From: Michael Grant <mgrant@grant.org>
Date: Wed, 5 Aug 2026 21:44:25 +0100
Subject: [PATCH] Fix SIXEL median-cut palette generation

---
 image-sixel.c            |  3 +++
 regress/image-support.sh | 22 ++++++++++++++++++++--
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/image-sixel.c b/image-sixel.c
index 01dc57fb0..dc1c4e93a 100644
--- a/image-sixel.c
+++ b/image-sixel.c
@@ -877,6 +877,9 @@ sixel_box_split(struct sixel_box *box, struct sixel_box *new,
 		if (count >= box->count / 2)
 			break;
 	}
+	/* Keep the maximum occupied level in the new box. */
+	if (level == last)
+		level--;
 	memcpy(new, box, sizeof *new);
 	if (channel == 0) {
 		box->red_max = level;
diff --git a/regress/image-support.sh b/regress/image-support.sh
index 8265d7933..8f3c6ca46 100755
--- a/regress/image-support.sh
+++ b/regress/image-support.sh
@@ -125,7 +125,7 @@ $TMUX2 select-window -t:1 || exit 1
 sleep 1
 [ "$($TMUX2 display-message -p '#{cursor_x},#{cursor_y}')" = "2,1" ] || exit 1
 $TMUX capture-pane -pS0 -E3 >$TMP || exit 1
-[ "$(sed -n 3p $TMP)" = "  :" ] || exit 1
+[ "$(sed -n 3p $TMP)" = "  -" ] || exit 1
 [ "$(sed -n 4p $TMP)" = "  *" ] || exit 1
 
 # With normal cursor movement, scrolling is calculated from the full image
@@ -139,7 +139,7 @@ $TMUX2 select-window -t:2 || exit 1
 sleep 1
 [ "$($TMUX2 display-message -p '#{cursor_y}')" = 3 ] || exit 1
 $TMUX capture-pane -pS0 -E3 >$TMP || exit 1
-[ "$(sed -n 1p $TMP)" = ":" ] || exit 1
+[ "$(sed -n 1p $TMP)" = "-" ] || exit 1
 [ "$(sed -n 2p $TMP)" = "*" ] || exit 1
 [ "$(sed -n 3p $TMP)" = "@" ] || exit 1
 
@@ -181,4 +181,22 @@ $TMUX capture-pane -pS0 -E3 >$TMP || exit 1
 [ "$(sed -n 1p $TMP | wc -c)" = 61 ] || exit 1
 [ -z "$(sed -n 2p $TMP)" ] || exit 1
 
+# A weighted median at the maximum channel level must still leave colours on
+# both sides of the split. This skewed black, grey and white image used to stop
+# palette generation after one colour instead of producing three.
+$TMUX kill-server 2>/dev/null
+$TMUX2 kill-server 2>/dev/null
+$TMUX2 new-session -d -x 20 -y 4 "
+	printf '\033_Ga=T,q=2,C=1,f=24,s=100,v=1,c=10,r=1;AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICA////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////\033\\'
+	sleep 10" || exit 1
+$TMUX2 set -g status off || exit 1
+$TMUX2 set -as terminal-features ',*:sixel' || exit 1
+$TMUX new-session -d -x 20 -y 4 || exit 1
+$TMUX set -g status off || exit 1
+$TMUX pipe-pane -O "cat >$TMP" || exit 1
+$TMUX send-keys -l "$TMUX2 attach-session" || exit 1
+$TMUX send-keys Enter || exit 1
+sleep 1
+grep -a '#2;2;' $TMP >/dev/null || exit 1
+
 exit 0