Fix to drop SIXEL cells that a newer image completely covers. And regression test added.

regress/image-sixel-overlap.sh

@@ -0,0 +1,79 @@
+#!/bin/sh
+
+# A SIXEL image drawn over an earlier one replaces the pixels underneath
+# for good, so an earlier image that is completely covered must not be kept
+# and drawn again on every repaint - an animation would otherwise pile up a
+# layer per frame.
+#
+# Draw two images over each other, then attach a client: it should be sent
+# no more than it would be for one image. An image that is only partly
+# covered must survive, so a second case draws a narrower image over a wider
+# one and expects both.
+
+PATH=/bin:/usr/bin
+TERM=screen
+LC_ALL=C.UTF-8
+export TERM LC_ALL
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -LtestA$$ -f/dev/null"
+TMUX2="$TEST_TMUX -LtestB$$ -f/dev/null"
+
+cleanup()
+{
+	$TMUX kill-server >/dev/null 2>&1
+	$TMUX2 kill-server >/dev/null 2>&1
+}
+fail()
+{
+	echo "$*" >&2
+	cleanup
+	exit 1
+}
+
+TMP=$(mktemp)
+trap "cleanup; rm -f $TMP" 0 1 15
+
+# Draw the given sixel commands in a new session, attach a sixel client and
+# set N to the number of SIXEL images the client was sent (other DCS
+# sequences, such as capability queries, do not count).
+count_images()
+{
+	cleanup
+	$TMUX new-session -d -s inner -x 40 -y 10 "printf '$1'; exec sh" ||
+		exit 1
+	sleep 0.3
+	[ "$($TMUX display-message -p '#{image_support}')" = 0 ] && exit 0
+	$TMUX set -as terminal-features ',*:sixel' || exit 1
+
+	$TMUX2 new-session -d -x 40 -y 10 || exit 1
+	OUTER=$($TMUX2 list-panes -F '#{pane_id}' | head -1)
+	[ -n "$OUTER" ] || fail "No outer pane."
+	$TMUX2 set -as terminal-features ',*:sixel@' ||
+	    fail "disable outer sixel failed"
+	: >"$TMP"
+	$TMUX2 pipe-pane -t "$OUTER" -O "cat >$TMP" || fail "pipe-pane failed"
+	$TMUX2 send-keys -t "$OUTER" -l "$TMUX attach -t inner" ||
+	    fail "send attach failed"
+	$TMUX2 send-keys -t "$OUTER" Enter || fail "send enter failed"
+	sleep 2
+	N=$(grep -aoE "$(printf '\033')P[0-9;]*q" "$TMP" | wc -l)
+}
+
+# One image on its own, as a baseline: a client is sent each image more than
+# once, as the terminal features are confirmed after attaching.
+count_images '\0337\033P9;1q#1!100~\033\\'
+SINGLE=$N
+[ "$SINGLE" -gt 0 ] || fail "sanity: client was sent no images"
+
+# The same footprint twice, blue then red.
+count_images '\0337\033P9;1q#1!100~\033\\\0338\033P9;1q#2!100~\033\\'
+[ "$N" -eq "$SINGLE" ] ||
+	fail "covered image was kept: client was sent $N images, expected $SINGLE"
+
+# A narrower image over a wider one leaves the wider one partly visible.
+count_images '\0337\033P9;1q#1!100~\033\\\0338\033P9;1q#2!40~\033\\'
+[ "$N" -gt "$SINGLE" ] ||
+	fail "partly covered image was lost: client was sent $N images, expected more than $SINGLE"
+
+exit 0