From c49dfdf496af9a0d835c37b2e9404b4284644c7e Mon Sep 17 00:00:00 2001
From: Nicholas Marriott <nicholas.marriott@gmail.com>
Date: Tue, 1 Sep 2026 20:51:20 +0100
Subject: [PATCH] Regress test for cursor updates.

---
 regress/sync-output-cursor.sh | 160 ++++++++++++++++++++++++++++++++++
 1 file changed, 160 insertions(+)
 create mode 100644 regress/sync-output-cursor.sh

diff --git a/regress/sync-output-cursor.sh b/regress/sync-output-cursor.sh
new file mode 100644
index 000000000..e805f373f
--- /dev/null
+++ b/regress/sync-output-cursor.sh
@@ -0,0 +1,160 @@
+#!/bin/sh
+
+# A synchronized update elsewhere in the pane must not hide a stationary
+# cursor while tmux waits for the end of the update.
+
+PATH=/bin:/usr/bin
+TERM=screen
+LC_ALL=C.UTF-8
+export PATH TERM LC_ALL
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+
+if [ "$#" -eq 0 ]; then
+	TEST_TMUX="$TEST_TMUX" sh "$0" pane || exit 1
+	TEST_TMUX="$TEST_TMUX" sh "$0" prompt || exit 1
+	exit 0
+fi
+case $1 in
+pane|prompt) MODE=$1 ;;
+*) echo "usage: $0 [pane|prompt]" >&2; exit 1 ;;
+esac
+
+DIR=$(mktemp -d) || exit 1
+TMUX_TMPDIR=$DIR
+export TMUX_TMPDIR
+
+INNER="$TEST_TMUX -Li$$ -f/dev/null"
+OUTER="$TEST_TMUX -Lo$$ -f/dev/null"
+CLIENT_BYTES=$DIR/client-bytes
+CONTROL=$DIR/control
+EMITTER=$DIR/emitter.pl
+ASSERT=$DIR/assert-cursor.pl
+
+fail()
+{
+	echo "$*" >&2
+	exit 1
+}
+
+cleanup()
+{
+	$OUTER kill-server 2>/dev/null
+	$INNER kill-server 2>/dev/null
+	rm -rf "$DIR"
+}
+trap cleanup 0 1 15
+
+wait_for_client()
+{
+	i=0
+	while [ "$i" -lt 50 ]; do
+		$INNER list-clients -F '#{client_termfeatures}' 2>/dev/null |
+		    grep -q 'sync' && return 0
+		sleep 0.1
+		i=$((i + 1))
+	done
+	fail "sync-capable client did not attach"
+}
+
+cat >"$EMITTER" <<'PERL'
+use strict;
+use warnings;
+
+my $control = $ENV{CONTROL};
+(my $dir = $control) =~ s{/[^/]+$}{};
+
+# Draw a prompt and leave its cursor stationary on the last line.
+syswrite STDOUT, "\e[2J\e[HWorking .\e[24;1H> ";
+open my $ready, '>', "$dir/ready" or die "$dir/ready: $!\n";
+close $ready;
+while (!-e $control) {
+	select undef, undef, undef, 0.01;
+}
+
+# Deliberately split BSU, content, and ESU across pane reads. This mirrors an
+# animation producer which begins a frame before it has finished rendering it.
+for my $spinner (qw(| / - \\)) {
+	syswrite STDOUT, "\e[?2026h";
+	select undef, undef, undef, 0.05;
+	syswrite STDOUT, "\e[1;9H$spinner\e[24;3H";
+	select undef, undef, undef, 0.01;
+	syswrite STDOUT, "\e[?2026l";
+	select undef, undef, undef, 0.15;
+}
+select undef, undef, undef, 5;
+PERL
+
+cat >"$ASSERT" <<'PERL'
+use strict;
+use warnings;
+
+my $path = shift;
+open my $fh, '<:raw', $path or die "$path: $!\n";
+local $/;
+my $bytes = <$fh>;
+my ($bsu, $esu, $hide, $show) =
+    ("\e[?2026h", "\e[?2026l", "\e[?25l", "\e[?25h");
+my ($sync, $outside, $hides, $shows) = (0, 0, 0, 0);
+
+for (my $i = 0; $i < length($bytes);) {
+	if (substr($bytes, $i, length($bsu)) eq $bsu) {
+		$sync++;
+		$i += length($bsu);
+		next;
+	}
+	if (substr($bytes, $i, length($esu)) eq $esu) {
+		$sync-- if $sync;
+		$i += length($esu);
+		next;
+	}
+	if (substr($bytes, $i, length($hide)) eq $hide) {
+		$hides++;
+		$outside++ unless $sync;
+		$i += length($hide);
+		next;
+	}
+	if (substr($bytes, $i, length($show)) eq $show) {
+		$shows++;
+		$outside++ unless $sync;
+		$i += length($show);
+		next;
+	}
+	$i++;
+}
+
+printf "%s cursor hides: %d; cursor shows: %d; changes outside sync: %d\n",
+    $ENV{MODE}, $hides, $shows, $outside;
+die "stationary cursor changed outside synchronized output\n" if $outside;
+PERL
+
+$INNER new-session -d -s inner -x 80 -y 24 \
+    "CONTROL='$CONTROL' perl '$EMITTER'" || exit 1
+$INNER set-option -g status off || exit 1
+$INNER set-option -g window-size manual || exit 1
+$INNER set-option -as terminal-features '*:sync' || exit 1
+
+$OUTER new-session -d -s outer -x 80 -y 24 \
+    "$TEST_TMUX -Li$$ -f/dev/null attach-session -t inner" || exit 1
+$OUTER set-option -g status off || exit 1
+$OUTER set-option -g window-size manual || exit 1
+wait_for_client
+
+if [ "$MODE" = prompt ]; then
+	$OUTER send-keys -t outer:0.0 C-b : || exit 1
+	sleep 0.2
+fi
+
+i=0
+while [ "$i" -lt 50 ] && [ ! -e "$DIR/ready" ]; do
+	sleep 0.1
+	i=$((i + 1))
+done
+[ -e "$DIR/ready" ] || fail "application emitter did not become ready"
+
+$OUTER pipe-pane -O -t outer:0.0 "cat >'$CLIENT_BYTES'" || exit 1
+: >"$CONTROL"
+sleep 2
+$OUTER pipe-pane -t outer:0.0 || exit 1
+
+MODE=$MODE perl "$ASSERT" "$CLIENT_BYTES"