diff --git a/regress/customize-mode.sh b/regress/customize-mode.sh
new file mode 100644
index 000000000..89ebc6a42
--- /dev/null
+++ b/regress/customize-mode.sh
@@ -0,0 +1,746 @@
+#!/bin/sh
+
+# Exercise customize-mode-specific mutations. mode-mutation.sh covers stale
+# mode-tree data while customize-mode is open; this covers the actions unique to
+# customize mode: adding, setting, editing, resetting, renaming array keys and
+# unsetting options, hooks, environment variables and key bindings.
+
+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$$ -f/dev/null"
+OUT="$TEST_TMUX -LtestB$$ -f/dev/null"
+FORMAT='CM #{?is_option,O,#{?is_environment,E,K}} #{option_name}#{environment_name}#{key}=#{option_value}#{environment_value}'
+
+cleanup_servers()
+{
+	$TMUX kill-server 2>/dev/null
+	$OUT kill-server 2>/dev/null
+	sleep 0.5
+}
+
+cleanup()
+{
+	cleanup_servers
+	rm -rf "$TMP"
+}
+trap cleanup EXIT
+
+fail()
+{
+	echo "$1" >&2
+	cleanup
+	exit 1
+}
+
+capture()
+{
+	$OUT capture-pane -p -t out:0 2>/dev/null
+}
+
+settle()
+{
+	sleep 0.3
+}
+
+wait_clients()
+{
+	i=0
+	while [ "$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()
+{
+	want=$1
+
+	i=0
+	while [ "$i" -lt 50 ]; do
+		got=$($TMUX display-message -p -t cm:0 '#{pane_in_mode}' \
+		    2>/dev/null)
+		[ "$got" = "$want" ] && return 0
+		sleep 0.2
+		i=$((i + 1))
+	done
+	fail "pane mode state is $got, expected $want"
+}
+
+assert_alive()
+{
+	$TMUX display-message -p 'alive' >/dev/null 2>&1 ||
+		fail "$1: server exited"
+}
+
+start_client()
+{
+	cleanup_servers
+	$TMUX new-session -d -s cm -n main -x 80 -y 24 'cat' ||
+		fail "new-session failed"
+	$OUT new-session -d -s out -x 80 -y 24 "$TMUX attach -t cm" ||
+		fail "outer client failed"
+	wait_clients 1
+}
+
+open_customize()
+{
+	if [ "$#" -eq 0 ]; then
+		$TMUX customize-mode -t cm:0 -F "$FORMAT" ||
+			fail "customize-mode failed"
+	else
+		$TMUX customize-mode -t cm:0 -F "$FORMAT" -f "$1" ||
+			fail "customize-mode failed"
+	fi
+	wait_mode 1
+	settle
+}
+
+send()
+{
+	$TMUX send-keys -t cm:0 "$@" || fail "send-keys failed: $*"
+}
+
+send_literal()
+{
+	$TMUX send-keys -t cm:0 -l "$1" || fail "send literal failed: $1"
+}
+
+repeat_key()
+{
+	key=$1
+	count=$2
+
+	i=0
+	while [ "$i" -lt "$count" ]; do
+		send "$key"
+		i=$((i + 1))
+	done
+}
+
+replace_prompt()
+{
+	send C-u
+	send_literal "$1"
+	send Enter
+	settle
+}
+
+assert_equals()
+{
+	got=$1
+	want=$2
+	what=$3
+
+	[ "$got" = "$want" ] ||
+		fail "$what: got '$got', expected '$want'"
+}
+
+assert_empty()
+{
+	got=$1
+	what=$2
+
+	[ -z "$got" ] || fail "$what: got '$got', expected empty"
+}
+
+assert_contains()
+{
+	text=$1
+	needle=$2
+	what=$3
+
+	echo "$text" | grep -F -q "$needle" ||
+		fail "$what: missing '$needle' in '$text'"
+}
+
+assert_not_contains()
+{
+	text=$1
+	needle=$2
+	what=$3
+
+	echo "$text" | grep -F -q "$needle" &&
+		fail "$what: found '$needle' in '$text'"
+	return 0
+}
+
+test_user_option()
+{
+	start_client
+
+	open_customize
+	send Enter
+	settle
+	send_literal "cm_add added"
+	send Enter
+	settle
+	assert_equals "$($TMUX show-options -sqv @cm_add)" "added" \
+		"added server user option"
+
+	send q
+	wait_mode 0
+	open_customize '#{==:#{option_name},@cm_add}'
+	send Right j Enter
+	settle
+	replace_prompt "changed"
+	assert_equals "$($TMUX show-options -sqv @cm_add)" "changed" \
+		"set user option"
+
+	send u y
+	settle
+	assert_empty "$($TMUX show-options -sqv @cm_add)" "unset user option"
+	send q
+	wait_mode 0
+}
+
+test_reset_window_pane_option()
+{
+	start_client
+
+	$TMUX set-option -g window-style fg=red ||
+		fail "set global window-style failed"
+	$TMUX set-window-option -t cm:0 window-style fg=green ||
+		fail "set window window-style failed"
+	$TMUX set-option -p -t cm:0.0 window-style fg=blue ||
+		fail "set pane window-style failed"
+
+	open_customize '#{==:#{option_name},window-style}'
+	send j j Right j
+	settle
+	send d y
+	settle
+
+	assert_equals "$($TMUX show-option -gqv window-style)" "default" \
+		"reset global window-style"
+	assert_empty "$($TMUX show-options -wqv -t cm:0 window-style)" \
+		"reset window window-style"
+	assert_empty "$($TMUX show-option -p -t cm:0.0 -qv window-style)" \
+		"reset pane window-style"
+	send q
+	wait_mode 0
+}
+
+test_environment()
+{
+	start_client
+
+	open_customize
+	repeat_key j 6
+	send Enter
+	settle
+	send_literal "CM_ENV=one"
+	send Enter
+	settle
+	assert_equals "$($TMUX show-environment -t cm CM_ENV)" \
+		"CM_ENV=one" "add session environment"
+
+	send q
+	wait_mode 0
+	open_customize '#{==:#{environment_name},CM_ENV}'
+	repeat_key j 6
+	send Right j Enter
+	settle
+	replace_prompt "two"
+	assert_equals "$($TMUX show-environment -t cm CM_ENV)" \
+		"CM_ENV=two" "set session environment"
+
+	send S
+	settle
+	replace_prompt "global"
+	assert_equals "$($TMUX show-environment -g CM_ENV)" \
+		"CM_ENV=global" "set global environment"
+
+	send u y
+	settle
+	$TMUX show-environment -t cm CM_ENV >/dev/null 2>&1 &&
+		fail "unset session environment: variable still exists"
+	send q
+	wait_mode 0
+}
+
+test_user_hook()
+{
+	start_client
+
+	open_customize
+	repeat_key j 3
+	send Enter
+	settle
+	send_literal "cm_hook display-message hook"
+	send Enter
+	settle
+	assert_equals "$($TMUX show-option -qv @cm_hook)" \
+		"display-message hook" "add user hook"
+
+	send q
+	wait_mode 0
+	open_customize '#{==:#{option_name},@cm_hook}'
+	repeat_key j 3
+	send Right j
+	settle
+	assert_contains "$(capture)" "@cm_hook" "user hook shown as hook"
+
+	send u y
+	settle
+	assert_empty "$($TMUX show-option -qv @cm_hook)" "unset user hook"
+	send q
+	wait_mode 0
+}
+
+test_hook_array()
+{
+	start_client
+
+	$TMUX set-hook -g 'after-new-session[0]' 'display-message old' ||
+		fail "set hook array failed"
+
+	open_customize '#{==:#{option_name},after-new-session}'
+	repeat_key j 3
+	send Right j Right j
+	settle
+
+	send a
+	settle
+	replace_prompt "1"
+	hooks=$($TMUX show-hooks -g after-new-session)
+	assert_contains "$hooks" "after-new-session[1] display-message old" \
+		"renamed hook array key"
+	assert_not_contains "$hooks" "after-new-session[0]" \
+		"old hook array key removed"
+
+	send Enter
+	settle
+	replace_prompt "display-message new"
+	hooks=$($TMUX show-hooks -g after-new-session)
+	assert_contains "$hooks" "after-new-session[1] display-message new" \
+		"set hook array value"
+
+	send u y
+	settle
+	hooks=$($TMUX show-hooks -g after-new-session)
+	assert_not_contains "$hooks" "after-new-session[1]" \
+		"unset hook array item"
+	send q
+	wait_mode 0
+}
+
+test_changed_only()
+{
+	start_client
+
+	$TMUX set-option -s @cm_changed yes ||
+		fail "set changed server option failed"
+	$TMUX bind-key -T prefix User1 display-message changed-only ||
+		fail "set changed key failed"
+
+	open_customize
+	send C
+	settle
+	send M-+
+	settle
+	screen=$(capture)
+	assert_contains "$screen" "@cm_changed" "changed-only user option"
+	assert_contains "$screen" "User1" "changed-only key"
+	assert_not_contains "$screen" "Global Environment" \
+		"changed-only hides environment"
+	send q
+	wait_mode 0
+}
+
+test_reset_key()
+{
+	start_client
+
+	orig=$($TMUX list-keys -T prefix -F '#{key_command}' c) ||
+		fail "default c binding not found"
+	$TMUX bind-key -T prefix c display-message changed ||
+		fail "change c binding failed"
+
+	open_customize '#{==:#{key},c}'
+	repeat_key j 10
+	send Right j d y
+	settle
+	after=$($TMUX list-keys -T prefix -F '#{key_command}' c) ||
+		fail "reset c binding not found"
+	assert_equals "$after" "$orig" "reset key binding"
+	send q
+	wait_mode 0
+}
+
+test_tagged_unset()
+{
+	start_client
+
+	$TMUX set-option -s @cm_tag_a one || fail "set tag option a failed"
+	$TMUX set-option -s @cm_tag_b two || fail "set tag option b failed"
+
+	open_customize '#{m/r:^@cm_tag_,#{option_name}}'
+	send Right j t j t U y
+	settle
+	assert_empty "$($TMUX show-options -sqv @cm_tag_a)" \
+		"tagged unset option a"
+	assert_empty "$($TMUX show-options -sqv @cm_tag_b)" \
+		"tagged unset option b"
+	send q
+	wait_mode 0
+}
+
+test_key_binding()
+{
+	start_client
+
+	open_customize
+	repeat_key j 10
+	send Enter
+	settle
+	send_literal "User0 display-message before"
+	send Enter
+	settle
+	keys=$($TMUX list-keys -T prefix User0 2>/dev/null) ||
+		fail "added key binding not found"
+	assert_contains "$keys" "display-message before" "add key binding"
+
+	send q
+	wait_mode 0
+	open_customize '#{==:#{key},User0}'
+	repeat_key j 10
+	send Right j Right j
+	settle
+
+	send s
+	settle
+	replace_prompt "display-message after"
+	keys=$($TMUX list-keys -T prefix User0 2>/dev/null) ||
+		fail "edited key binding not found"
+	assert_contains "$keys" "display-message after" "set key command"
+
+	send j s
+	settle
+	replace_prompt "custom note"
+	note=$($TMUX list-keys -T prefix -F '#{key_note}' User0)
+	assert_equals "$note" "custom note" "set key note"
+
+	send j s
+	settle
+	keys=$($TMUX list-keys -T prefix User0 2>/dev/null) ||
+		fail "repeat key binding not found"
+	assert_contains "$keys" "bind-key -r" "toggle key repeat"
+
+	send u y
+	settle
+	$TMUX list-keys -T prefix User0 >/dev/null 2>&1 &&
+		fail "unset key binding: binding still exists"
+	send q
+	wait_mode 0
+}
+
+test_editor()
+{
+	start_client
+
+	editor="$TMP/editor.sh"
+	editor_value="$TMP/editor-value"
+	{
+		echo '#!/bin/sh'
+		printf 'cat "%s" > "$1"\n' "$editor_value"
+	} > "$editor" || fail "write editor helper failed"
+	chmod +x "$editor" || fail "chmod editor helper failed"
+
+	$TMUX set-option -g editor "$editor" || fail "set editor failed"
+	$TMUX set-option -g @cm_edit old || fail "set edit option failed"
+
+	printf 'edited' > "$editor_value" || fail "write editor value failed"
+	open_customize '#{==:#{option_name},@cm_edit}'
+	send j Right j e
+
+	i=0
+	while [ "$i" -lt 50 ]; do
+		value=$($TMUX show-option -gqv @cm_edit)
+		[ "$value" = "edited" ] && break
+		sleep 0.2
+		i=$((i + 1))
+	done
+	assert_equals "$value" "edited" "edit option in editor"
+	send q
+	wait_mode 0
+
+	$TMUX bind-key -T prefix User2 display-message old ||
+		fail "set editor key failed"
+	printf 'display-message edited' > "$editor_value" ||
+		fail "write editor key command failed"
+	open_customize '#{==:#{key},User2}'
+	repeat_key j 10
+	send Right j Right j e
+	i=0
+	while [ "$i" -lt 50 ]; do
+		value=$($TMUX list-keys -T prefix -F '#{key_command}' User2)
+		[ "$value" = "display-message edited" ] && break
+		sleep 0.2
+		i=$((i + 1))
+	done
+	assert_equals "$value" "display-message edited" \
+		"edit key command in editor"
+
+	printf 'edited note' > "$editor_value" ||
+		fail "write editor key note failed"
+	send j e
+	i=0
+	while [ "$i" -lt 50 ]; do
+		value=$($TMUX list-keys -T prefix -F '#{key_note}' User2)
+		[ "$value" = "edited note" ] && break
+		sleep 0.2
+		i=$((i + 1))
+	done
+	assert_equals "$value" "edited note" "edit key note in editor"
+	send q
+	wait_mode 0
+
+	$TMUX set-environment -t cm CM_EDIT_ENV old ||
+		fail "set editor environment failed"
+	printf 'env-edited' > "$editor_value" ||
+		fail "write editor environment failed"
+	open_customize '#{==:#{environment_name},CM_EDIT_ENV}'
+	repeat_key j 6
+	send Right j e
+	i=0
+	while [ "$i" -lt 50 ]; do
+		value=$($TMUX show-environment -t cm CM_EDIT_ENV)
+		[ "$value" = "CM_EDIT_ENV=env-edited" ] && break
+		sleep 0.2
+		i=$((i + 1))
+	done
+	assert_equals "$value" "CM_EDIT_ENV=env-edited" \
+		"edit environment in editor"
+	send q
+	wait_mode 0
+}
+
+test_stale_current_items()
+{
+	start_client
+
+	$TMUX set-option -s @cm_stale_option old ||
+		fail "set stale option failed"
+	open_customize '#{==:#{option_name},@cm_stale_option}'
+	send Right j
+	settle
+	$TMUX set-option -su @cm_stale_option ||
+		fail "external unset stale option failed"
+	send Enter
+	send u y
+	send d y
+	settle
+	assert_alive "stale option current item"
+	send q
+	wait_mode 0
+
+	$TMUX set-environment -t cm CM_STALE_ENV old ||
+		fail "set stale environment failed"
+	open_customize '#{==:#{environment_name},CM_STALE_ENV}'
+	repeat_key j 6
+	send Right j
+	settle
+	$TMUX set-environment -t cm -u CM_STALE_ENV ||
+		fail "external unset stale environment failed"
+	send Enter
+	send S
+	send u y
+	settle
+	assert_alive "stale environment current item"
+	send q
+	wait_mode 0
+
+	$TMUX bind-key -T prefix User3 display-message stale ||
+		fail "set stale key failed"
+	open_customize '#{==:#{key},User3}'
+	repeat_key j 10
+	send Right j
+	settle
+	$TMUX unbind-key -T prefix User3 ||
+		fail "external unbind stale key failed"
+	send s
+	send d y
+	send u y
+	settle
+	assert_alive "stale key current item"
+	send q
+	wait_mode 0
+}
+
+test_prompt_invalidation()
+{
+	start_client
+
+	$TMUX set-option -s @cm_prompt_stale old ||
+		fail "set prompt stale option failed"
+	open_customize '#{==:#{option_name},@cm_prompt_stale}'
+	send Right j Enter
+	settle
+	$TMUX set-option -su @cm_prompt_stale ||
+		fail "external unset prompt option failed"
+	replace_prompt "new"
+	assert_empty "$($TMUX show-options -sqv @cm_prompt_stale)" \
+		"prompt stale option should not be recreated"
+	assert_alive "stale option prompt"
+	send q
+	wait_mode 0
+
+	$TMUX bind-key -T prefix User4 display-message old ||
+		fail "set prompt stale key failed"
+	open_customize '#{==:#{key},User4}'
+	repeat_key j 10
+	send Right j Right j s
+	settle
+	$TMUX unbind-key -T prefix User4 ||
+		fail "external unbind prompt key failed"
+	replace_prompt "display-message new"
+	$TMUX list-keys -T prefix User4 >/dev/null 2>&1 &&
+		fail "prompt stale key should stay unbound"
+	assert_alive "stale key prompt"
+	send q
+	wait_mode 0
+
+	$TMUX set-hook -g 'after-new-session[0]' 'display-message old' ||
+		fail "set prompt stale hook array failed"
+	$TMUX set-hook -g 'after-new-session[1]' 'display-message taken' ||
+		fail "set prompt stale hook array target failed"
+	open_customize '#{==:#{option_name},after-new-session}'
+	repeat_key j 3
+	send Right j Right j a
+	settle
+	replace_prompt "1"
+	hooks=$($TMUX show-hooks -g after-new-session)
+	assert_contains "$hooks" "after-new-session[0] display-message old" \
+		"array rename to existing key keeps old key"
+	assert_contains "$hooks" "after-new-session[1] display-message taken" \
+		"array rename to existing key keeps target key"
+	assert_alive "existing array key prompt"
+	send q
+	wait_mode 0
+}
+
+test_editor_invalidation()
+{
+	start_client
+
+	editor="$TMP/slow-editor.sh"
+	{
+		echo '#!/bin/sh'
+		echo 'sleep 1'
+		echo 'printf edited > "$1"'
+	} > "$editor" || fail "write slow editor helper failed"
+	chmod +x "$editor" || fail "chmod slow editor helper failed"
+	$TMUX set-option -g editor "$editor" || fail "set slow editor failed"
+
+	$TMUX set-option -g @cm_editor_stale old ||
+		fail "set editor stale option failed"
+	open_customize '#{==:#{option_name},@cm_editor_stale}'
+	send j Right j e
+	settle
+	$TMUX set-option -gu @cm_editor_stale ||
+		fail "external unset editor option failed"
+	sleep 2
+	assert_empty "$($TMUX show-option -gqv @cm_editor_stale)" \
+		"editor stale option should not be recreated"
+	assert_alive "stale option editor"
+	send q
+	wait_mode 0
+
+	$TMUX bind-key -T prefix User5 display-message old ||
+		fail "set editor stale key failed"
+	open_customize '#{==:#{key},User5}'
+	repeat_key j 10
+	send Right j Right j e
+	settle
+	$TMUX unbind-key -T prefix User5 ||
+		fail "external unbind editor key failed"
+	sleep 2
+	$TMUX list-keys -T prefix User5 >/dev/null 2>&1 &&
+		fail "editor stale key should stay unbound"
+	assert_alive "stale key editor"
+	send q
+	wait_mode 0
+
+	$TMUX set-environment -t cm CM_EDITOR_STALE old ||
+		fail "set editor stale environment failed"
+	open_customize '#{==:#{environment_name},CM_EDITOR_STALE}'
+	repeat_key j 6
+	send Right j e
+	settle
+	$TMUX set-environment -t cm -u CM_EDITOR_STALE ||
+		fail "external unset editor environment failed"
+	sleep 2
+	assert_equals "$($TMUX show-environment -t cm CM_EDITOR_STALE)" \
+		"CM_EDITOR_STALE=edited" "editor stale environment is reset"
+	assert_alive "stale environment editor"
+	send q
+	wait_mode 0
+}
+
+test_invalid_prompts()
+{
+	start_client
+
+	open_customize
+	send Enter
+	settle
+	send_literal "bad value"
+	send Enter
+	settle
+	assert_empty "$($TMUX show-options -sqv bad)" "invalid user option name"
+	assert_alive "invalid user option prompt"
+	send q
+	wait_mode 0
+
+	open_customize
+	repeat_key j 10
+	send Enter
+	settle
+	send_literal "NoSuchKey display-message invalid"
+	send Enter
+	settle
+	assert_alive "invalid key prompt"
+	send q
+	wait_mode 0
+
+	$TMUX bind-key -T prefix User6 display-message valid ||
+		fail "set invalid command key failed"
+	open_customize '#{==:#{key},User6}'
+	repeat_key j 10
+	send Right j Right j s
+	settle
+	replace_prompt "not-a-command -Z"
+	keys=$($TMUX list-keys -T prefix -F '#{key_command}' User6)
+	assert_equals "$keys" "display-message valid" \
+		"invalid key command keeps old command"
+	assert_alive "invalid key command prompt"
+	send q
+	wait_mode 0
+}
+
+cleanup_servers
+test_user_option
+test_reset_window_pane_option
+test_environment
+test_user_hook
+test_hook_array
+test_changed_only
+test_reset_key
+test_tagged_unset
+test_key_binding
+test_editor
+test_stale_current_items
+test_prompt_invalidation
+test_editor_invalidation
+test_invalid_prompts
+cleanup
+exit 0