Add a test for Kitty keyboard protocol input parsing

Also note that kitty-keys-terminal.sh cannot run under the regress
Makefile, which uses env -i and so removes DISPLAY and HOME, and make the
Kitty test file modes match the rest of regress.

regress/kitty-keys-input.sh

@@ -0,0 +1,119 @@
+#!/bin/sh
+
+# Test client-side Kitty keyboard protocol support: an outer tmux acts as a
+# Kitty-capable terminal for the inner tmux which is the one under test. The
+# inner tmux queries the outer one, the outer one replies, and the inner one
+# then parses the key sequences sent to it.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+CONF=$(mktemp)
+TMP=$(mktemp)
+
+printf '%s\n' 'set -g extended-keys on' \
+    'set -g extended-keys-format kitty' >"$CONF"
+
+TMUX="$TEST_TMUX -LtestKIA$$ -f$CONF"
+TMUX2="$TEST_TMUX -LtestKIB$$ -f$CONF"
+
+trap 'rm -f "$CONF" "$TMP"; $TMUX kill-server 2>/dev/null; \
+    $TMUX2 kill-server 2>/dev/null' 0 1 15
+
+$TMUX2 new-session -d -x80 -y24 || exit 1
+$TMUX new-session -d -x80 -y24 "$TMUX2 attach" || exit 1
+
+exit_status=0
+
+wait_for_mode()
+{
+	i=0
+	while [ "$($TMUX display-message -pt: '#{pane_key_mode}')" != "$1" ] &&
+	    [ "$i" -lt 50 ]; do
+		sleep 0.1
+		i=$((i + 1))
+	done
+	[ "$($TMUX display-message -pt: '#{pane_key_mode}')" = "$1" ]
+}
+
+assert_key()
+{
+	keys=$1
+	expected=$2
+
+	$TMUX2 command-prompt -k 'display-message -pl "%%"' >"$TMP" &
+	sleep 0.15
+	$TMUX send-keys $keys
+	wait
+
+	actual=$(tr -d '[:space:]' <"$TMP")
+	if [ "$actual" = "$expected" ]; then
+		[ -n "$VERBOSE" ] && echo "[PASS] $keys -> $actual"
+	else
+		echo "[FAIL] $keys -> $expected (got '$actual')"
+		exit_status=1
+	fi
+}
+
+# The inner tmux must detect the outer one and push its flags.
+if ! wait_for_mode 'Kitty 1'; then
+	echo "[FAIL] inner tmux did not enable the Kitty protocol"
+	exit 1
+fi
+
+# The handshake is done, so stop the outer tmux encoding the raw sequences
+# below as Kitty keys itself.
+$TMUX set-option -g extended-keys-format csi-u
+
+# Modifiers, including the Kitty-only Super and Hyper.
+assert_key 'Escape [97;2u' 'S-a'
+assert_key 'Escape [97;3u' 'M-a'
+assert_key 'Escape [97;5u' 'C-a'
+assert_key 'Escape [97;6u' 'C-S-a'
+assert_key 'Escape [97;7u' 'C-M-a'
+assert_key 'Escape [97;9u' 's-a'
+assert_key 'Escape [97;17u' 'H-a'
+assert_key 'Escape [65;2u' 'S-A'
+
+# Keys with a dedicated tmux key code.
+assert_key 'Escape [9;5u' 'C-Tab'
+assert_key 'Escape [9;2u' 'BTab'
+assert_key 'Escape [13;5u' 'C-Enter'
+assert_key 'Escape [27u' 'Escape'
+assert_key 'Escape [32;5u' 'C-Space'
+assert_key 'Escape [127;5u' 'C-BSpace'
+
+# Keys with a CSI final byte other than u.
+assert_key 'Escape [1;5A' 'C-Up'
+assert_key 'Escape [1;3D' 'M-Left'
+assert_key 'Escape [1;5H' 'C-Home'
+assert_key 'Escape [1;5P' 'C-F1'
+
+# Functional and non-ASCII keys.
+assert_key 'Escape [57399;5u' 'C-KP0'
+assert_key 'Escape [233;5u' 'C-é'
+
+# The alternate key and associated text fields are not requested but must be
+# ignored if a terminal sends them anyway.
+assert_key 'Escape [97:65;2u' 'S-a'
+assert_key 'Escape [97:65:97;5u' 'C-a'
+assert_key 'Escape [97;5;97u' 'C-a'
+
+# A repeat event is a key press, a release event is dropped. The C-b that
+# follows shows that the sequence was consumed and not passed through.
+assert_key 'Escape [97;5:1u' 'C-a'
+assert_key 'Escape [97;5:2u' 'C-a'
+assert_key 'Escape [97;5:3u Escape [98;5u' 'C-b'
+
+# The Meta modifier bit is not representable and is dropped.
+assert_key 'Escape [97;33u Escape [98;5u' 'C-b'
+
+# Lock modifiers are ignored rather than dropping the key.
+assert_key 'Escape [97;65u' 'a'
+assert_key 'Escape [97;129u' 'a'
+
+$TMUX kill-server 2>/dev/null
+$TMUX2 kill-server 2>/dev/null
+
+exit $exit_status

regress/kitty-keys-terminal.sh

@@ -2,8 +2,12 @@
 
 # Test the Kitty keyboard protocol against a real terminal rather than against
 # another copy of tmux. This drives kitty through its remote control interface
-# so that kitty itself encodes the key presses. It is skipped unless kitty, a
-# display and a kitty new enough to have send-key are all available.
+# so that kitty itself encodes the key presses.
+#
+# This has to be run by hand from this directory. The Makefile runs tests with
+# env -i, which removes DISPLAY and HOME, so kitty cannot start and this exits
+# immediately without testing anything. It is also skipped unless kitty is
+# installed and is new enough to have send-key.
 
 PATH=/bin:/usr/bin