diff --git a/regress/mode-kill.sh b/regress/mode-kill.sh
new file mode 100755
index 000000000..a3e0d09b9
--- /dev/null
+++ b/regress/mode-kill.sh
@@ -0,0 +1,171 @@
+#!/bin/sh
+
+# Tests of the -k flag on the mode-entering commands (cmd-copy-mode.c and
+# cmd-choose-tree.c). With -k the pane is killed when the mode is exited: this
+# is stored on the mode entry in window_pane_set_mode() and acted on in
+# window_pane_reset_mode() (window.c). It is exercised here for:
+#
+# - copy-mode -k (window-copy.c);
+# - choose-tree -k (window-tree.c);
+# - choose-buffer -k (window-buffer.c).
+#
+# choose-tree and choose-buffer share cmd_choose_tree_exec(), which also backs
+# choose-client and customize-mode, so those are not repeated.
+#
+# Each mode is entered in the active pane of a two-pane window: exiting with
+# -k must remove that pane and leave the other. copy-mode is left with the
+# server-side "-X cancel" and needs no client, so those tests run first. The
+# tree modes only act on a key once the client has drawn the mode, so - as in
+# choose-tree.sh - a second server then provides a client (an inner "tmux
+# attach") and its pane is captured to wait until the mode has rendered before
+# the exit key is sent.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMP=$(mktemp -d) || exit 1
+TMUX_TMPDIR="$TMP"
+export TMUX_TMPDIR
+TMUX="$TEST_TMUX -LtestA$$ -f/dev/null"
+TMUX2="$TEST_TMUX -LtestB$$ -f/dev/null"
+
+cleanup()
+{
+	$TMUX kill-server 2>/dev/null
+	$TMUX2 kill-server 2>/dev/null
+	rm -rf "$TMP"
+}
+trap cleanup EXIT
+
+fail()
+{
+	echo "$1" >&2
+	exit 1
+}
+
+# capture the screen rendered by the inner client.
+capture()
+{
+	$TMUX2 capture-pane -p -t out:0 2>/dev/null
+}
+
+# wait_clients $n: wait until the test server has exactly $n clients.
+wait_clients()
+{
+	i=0
+	while [ "$i" -lt 50 ]; do
+		c=$($TMUX list-clients -F x 2>/dev/null | grep -c x)
+		[ "$c" -eq "$1" ] && return 0
+		sleep 0.2
+		i=$((i + 1))
+	done
+	fail "expected $1 clients, have $c"
+}
+
+# wait_mode $target $state: wait until a pane enters (1) or leaves (0) mode.
+wait_mode()
+{
+	i=0
+	while [ "$i" -lt 50 ]; do
+		got=$($TMUX display-message -p -t "$1" '#{pane_in_mode}' \
+		    2>/dev/null)
+		[ "$got" = "$2" ] && return 0
+		sleep 0.2
+		i=$((i + 1))
+	done
+	fail "pane $1 mode state is '$got', expected '$2'"
+}
+
+# wait_for $marker: wait until the rendered screen contains $marker.
+wait_for()
+{
+	i=0
+	while [ "$i" -lt 50 ]; do
+		capture | grep -F -q "$1" && return 0
+		sleep 0.2
+		i=$((i + 1))
+	done
+	fail "timed out waiting for '$1' to render"
+}
+
+# pane_gone $pane-id: true if the pane no longer exists. (display-message -t
+# on a missing pane id falls back to a default target and succeeds, so the
+# pane list is searched instead.)
+pane_gone()
+{
+	! $TMUX list-panes -s -t m -F '#{pane_id}' 2>/dev/null | \
+	    grep -q -x "$1"
+}
+
+# open_window: make a fresh two-pane window and leave the new (active) pane
+# as $active and the other as $other.
+open_window()
+{
+	$TMUX new-window -t m: -n w 'cat' || fail "new-window failed"
+	$TMUX split-window -t m:w 'cat' || fail "split-window failed"
+	other=$($TMUX display-message -p -t m:w.0 '#{pane_id}')
+	active=$($TMUX display-message -p -t m:w '#{pane_id}')
+}
+
+# check_killed $label: wait for the active pane to be killed, leaving only the
+# other pane, then drop the window.
+check_killed()
+{
+	i=0
+	while [ "$i" -lt 50 ]; do
+		pane_gone "$active" && break
+		sleep 0.2
+		i=$((i + 1))
+	done
+	pane_gone "$active" || fail "$1: pane not killed on exit"
+	panes=$($TMUX list-panes -t m:w -F '#{pane_id}' | tr '\n' ' ')
+	[ "$panes" = "$other " ] || \
+	    fail "$1: expected only $other left, have $panes"
+	$TMUX kill-window -t m:w 2>/dev/null
+}
+
+# Session m; window 0 keeps a live pane so the session (and later the client)
+# survives each test killing a pane.
+$TMUX new-session -d -s m -x 80 -y 24 'cat' || exit 1
+
+# --- copy-mode -k kills the pane, plain copy-mode does not -------------------
+#
+# copy-mode is exited with the server-side "-X cancel", so no client is needed
+# and none is attached yet.
+open_window
+$TMUX copy-mode -k -t m:w || fail "copy-mode -k failed"
+wait_mode "$active" 1
+$TMUX send-keys -t m:w -X cancel || fail "copy cancel failed"
+check_killed 'copy-mode -k'
+
+open_window
+$TMUX copy-mode -t m:w || fail "copy-mode failed"
+wait_mode "$active" 1
+$TMUX send-keys -t m:w -X cancel || fail "copy cancel failed"
+wait_mode "$active" 0
+pane_gone "$active" && fail 'copy-mode: pane killed without -k'
+$TMUX kill-window -t m:w 2>/dev/null
+
+# --- choose-tree -k and choose-buffer -k kill the pane ----------------------
+#
+# These need the client to draw the mode before a key acts, so attach one now.
+# A paste buffer is needed for choose-buffer to have something to show, and a
+# distinct -F marker per call is waited for in the capture so the exit key is
+# only sent once the mode is drawn.
+$TMUX set-buffer 'mode-kill buffer' || exit 1
+$TMUX2 new-session -d -s out -x 80 -y 24 "$TMUX attach -t m" || exit 1
+wait_clients 1
+open_window
+$TMUX choose-tree -k -F 'TREEMARK' -t m:w || fail "choose-tree -k failed"
+wait_for 'TREEMARK'
+$TMUX send-keys -t m:w q || fail "choose-tree exit failed"
+check_killed 'choose-tree -k'
+
+open_window
+$TMUX choose-buffer -k -F 'BUFMARK' -t m:w || fail "choose-buffer -k failed"
+wait_for 'BUFMARK'
+$TMUX send-keys -t m:w q || fail "choose-buffer exit failed"
+check_killed 'choose-buffer -k'
+
+exit 0
diff --git a/regress/pane-ops.sh b/regress/pane-ops.sh
index 762bea200..29c79e2e2 100644
--- a/regress/pane-ops.sh
+++ b/regress/pane-ops.sh
@@ -399,6 +399,19 @@ check_fmt 'P:9.0' '#{pane_pid}' ''
 check_fail 'command cannot be given for empty pane' \
     new-window -d -E -t P:10 -n empty 'true'
 
+# An empty string as the sole argument is equivalent to -E: the pane is
+# created empty, running no command.
+check_ok new-window -d -t P:12 -n empty-str ''
+check_fmt 'P:12.0' '#{pane_dead}' '0'
+check_fmt 'P:12.0' '#{pane_pid}' ''
+check_ok kill-window -t P:12
+
+# A missing command (rather than an empty one) runs the default command, so
+# the pane is not empty and has a process.
+check_ok new-window -d -t P:12 -n default-cmd
+check_fmt 'P:12.0' '#{?pane_pid,live,empty}' 'live'
+check_ok kill-window -t P:12
+
 # respawn-pane -E stores the command and cwd without starting it.
 tmp=${TMPDIR:-/tmp}/tmux-pane-ops-empty-$$
 rm -f "$tmp"
@@ -453,6 +466,13 @@ check_fmt 'P:2' '#{window_panes}' '2'
 check_fail 'command cannot be given for empty pane' \
 	split-window -d -E -t P:2.0 'sleep 5'
 
+# An empty string as the sole argument splits with an empty pane, like -E.
+eid=$($TMUX split-window -d -P -F '#{pane_id}' -t P:2.0 '')
+check_fmt "$eid" '#{pane_dead}' '0'
+check_fmt "$eid" '#{pane_pid}' ''
+check_ok kill-pane -t "$eid"
+check_fmt 'P:2' '#{window_panes}' '2'
+
 # -e adds to the new pane's environment.
 eid=$($TMUX split-window -d -P -F '#{pane_id}' -e GREETING=hello -t P:2.0 \
 	'echo $GREETING; exec cat')