From 7bd1ba9cad3c900289768ce2d099f2de145f64e2 Mon Sep 17 00:00:00 2001
From: Michael Grant <mgrant@grant.org>
Date: Mon, 10 Aug 2026 15:12:47 +0100
Subject: [PATCH] sixel: preserve fractional raster extents

---
 image-sixel.c            | 25 ++++++++++++++++++++++---
 regress/image-support.sh | 10 ++++++++++
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/image-sixel.c b/image-sixel.c
index 324f46b65..f687a51d8 100644
--- a/image-sixel.c
+++ b/image-sixel.c
@@ -602,7 +602,7 @@ sixel_scale(struct sixel_image *si, u_int cell_w, u_int cell_h, u_int ox,
     u_int oy, u_int sx, u_int sy, int colours)
 {
 	struct sixel_image	*new;
-	uint64_t		 x0, x1, y0, y1;
+	uint64_t		 x0, x1, y0, y1, tx0, tx1, ty0, ty1;
 	u_int			 cx, cy, pox, poy, psx, psy, tsx, tsy, px, py;
 	u_int			 x, y, i;
 
@@ -640,8 +640,27 @@ sixel_scale(struct sixel_image *si, u_int cell_w, u_int cell_h, u_int ox,
 	psx = x1 - x0;
 	psy = y1 - y0;
 
-	tsx = sx * cell_w;
-	tsy = sy * cell_h;
+	/*
+	 * Preserve any partial final source cell. The grid still covers whole
+	 * cells, but the SIXEL raster must end at the corresponding pixel offset
+	 * rather than stretching to the cell boundary.
+	 */
+	tx1 = ((uint64_t)si->sx * cell_w + si->cell_w - 1) / si->cell_w;
+	ty1 = ((uint64_t)si->sy * cell_h + si->cell_h - 1) / si->cell_h;
+	if (tx1 > UINT_MAX || ty1 > UINT_MAX)
+		return (NULL);
+	tx0 = (uint64_t)ox * cell_w;
+	ty0 = (uint64_t)oy * cell_h;
+	if (tx0 >= tx1 || ty0 >= ty1)
+		return (NULL);
+	if ((uint64_t)(ox + sx) * cell_w < tx1)
+		tx1 = (uint64_t)(ox + sx) * cell_w;
+	if ((uint64_t)(oy + sy) * cell_h < ty1)
+		ty1 = (uint64_t)(oy + sy) * cell_h;
+	tsx = tx1 - tx0;
+	tsy = ty1 - ty0;
+	if (tsx == 0 || tsy == 0)
+		return (NULL);
 
 	new = xcalloc (1, sizeof *si);
 	new->cell_w = cell_w;
diff --git a/regress/image-support.sh b/regress/image-support.sh
index 41e60e191..c15ec30bb 100755
--- a/regress/image-support.sh
+++ b/regress/image-support.sh
@@ -50,14 +50,24 @@ $TMUX2 new-session -d -x 10 -y 4 "
 	printf '\033Pq\"1;1;26;26#0;2;100;100;100#0!26~-!26~-!26~-!26~-!26B\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 10 -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
 $TMUX capture-pane -pS0 -E0 >$TMP || exit 1
 grep -q '^#=' $TMP || exit 1
 
+# Re-emitting the image preserves the 26-pixel raster rather than expanding
+# it to the two complete 16-pixel grid cells occupied by the image.
+$TMUX pipe-pane || exit 1
+$TMUX pipe-pane -O "cat >$TMP" || exit 1
+$TMUX2 refresh-client -R || exit 1
+sleep 1
+grep -a '"1;1;26;26' $TMP >/dev/null || exit 1
+
 # Selection redraws must leave text image cells visible.
 $TMUX2 copy-mode || exit 1
 $TMUX2 send-keys -X history-top || exit 1