diff --git a/regress/sync-output-atomic.sh b/regress/sync-output-atomic.sh
new file mode 100644
index 000000000..d0979ebe1
--- /dev/null
+++ b/regress/sync-output-atomic.sh
@@ -0,0 +1,173 @@
+#!/bin/sh
+
+# Application synchronized updates must remain one physical client transaction,
+# including when BSU, printable cells and ESU arrive in one pane read.
+
+PATH=/bin:/usr/bin
+TERM=screen
+LC_ALL=C.UTF-8
+export PATH TERM LC_ALL
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+
+DIR=$(mktemp -d) || exit 1
+TMUX_TMPDIR=$DIR
+export TMUX_TMPDIR
+
+INNER="$TEST_TMUX -Lsync-output-inner-$$ -f/dev/null"
+OUTER="$TEST_TMUX -Lsync-output-outer-$$ -f/dev/null"
+CLIENT_BYTES=$DIR/client-bytes
+CONTROL=$DIR/control
+EMITTER=$DIR/emitter.pl
+ASSERT=$DIR/assert-sync-output.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"
+}
+
+wait_for_marker()
+{
+	i=0
+	while [ "$i" -lt 50 ]; do
+		grep -q 'FRAME_000001_ROW_23' "$CLIENT_BYTES" 2>/dev/null && return 0
+		sleep 0.1
+		i=$((i + 1))
+	done
+	fail "client did not receive both synchronized frames"
+}
+
+wait_for_stable_bytes()
+{
+	previous=-1
+	stable=0
+	i=0
+	while [ "$i" -lt 50 ]; do
+		current=$(wc -c <"$CLIENT_BYTES" 2>/dev/null) || current=0
+		if [ "$current" -gt 0 ] && [ "$current" -eq "$previous" ]; then
+			stable=$((stable + 1))
+			[ "$stable" -eq 5 ] && return 0
+		else
+			stable=0
+		fi
+		previous=$current
+		sleep 0.1
+		i=$((i + 1))
+	done
+	fail "client byte stream did not become stable"
+}
+
+cat >"$EMITTER" <<'PERL'
+use strict;
+use warnings;
+
+my $control = $ENV{CONTROL};
+(my $dir = $control) =~ s{/[^/]+$}{};
+open my $ready, '>', "$dir/ready" or die "$dir/ready: $!\n";
+close $ready;
+while (!-e $control) {
+	select undef, undef, undef, 0.01;
+}
+for my $frame (0 .. 1) {
+	my @rows;
+	for my $row (0 .. 23) {
+		my $marker = sprintf "FRAME_%06d_ROW_%02d_", $frame, $row;
+		push @rows, substr($marker . ('X' x 79), 0, 79);
+	}
+	my $frame = "\e[?2026h\e[H" . join("\r\n", @rows) .
+	    "\e[?2026l";
+	my $written = syswrite STDOUT, $frame;
+	die "short synchronized frame write\n"
+	    unless defined $written && $written == length $frame;
+	select undef, undef, undef, 0.2;
+}
+select undef, undef, undef, 2;
+PERL
+
+$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 -Lsync-output-inner-$$ -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
+
+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
+: >"$CONTROL"
+wait_for_marker
+wait_for_stable_bytes
+$OUTER pipe-pane -t outer:0.0 || exit 1
+
+cat >"$ASSERT" <<'PERL'
+use strict;
+use warnings;
+
+my $path = shift;
+open my $fh, '<:raw', $path or die "$path: $!\n";
+local $/;
+my $bytes = <$fh>;
+my ($on, $off) = ("\e[?2026h", "\e[?2026l");
+my ($active, %seen);
+
+for (my $i = 0; $i < length($bytes);) {
+	if (substr($bytes, $i, length($on)) eq $on) {
+		die "duplicate synchronized-output start at byte $i\n" if $active;
+		$active = 1;
+		$i += length($on);
+		next;
+	}
+	if (substr($bytes, $i, length($off)) eq $off) {
+		die "unmatched synchronized-output end at byte $i\n" unless $active;
+		$active = 0;
+		$i += length($off);
+		next;
+	}
+	if (substr($bytes, $i) =~ /\AFRAME_(\d{6})_ROW_(\d{2})_/) {
+		my $marker = "$1:$2";
+		die "marker $marker outside synchronized output\n" unless $active;
+		$seen{$marker}++;
+	}
+	$i++;
+}
+die "unterminated synchronized-output transaction\n" if $active;
+for my $frame (0 .. 1) {
+	for my $row (0 .. 23) {
+		my $marker = sprintf "%06d:%02d", $frame, $row;
+		die "missing marker $marker\n" unless $seen{$marker};
+	}
+}
+PERL
+
+perl "$ASSERT" "$CLIENT_BYTES" || exit 1