diff --git a/regress/new-pane-mouse.sh b/regress/new-pane-mouse.sh
new file mode 100644
index 000000000..d2ae9ee97
--- /dev/null
+++ b/regress/new-pane-mouse.sh
@@ -0,0 +1,81 @@
+#!/bin/sh
+
+# Test new-pane -M creates a floating pane from a mouse drag.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -LtestA$$ -f/dev/null"
+TMUX2="$TEST_TMUX -LtestB$$ -f/dev/null"
+
+cleanup()
+{
+	$TMUX kill-server >/dev/null 2>&1
+	$TMUX2 kill-server >/dev/null 2>&1
+}
+fail()
+{
+	echo "$*" >&2
+	cleanup
+	exit 1
+}
+must_equal()
+{
+	got=$1
+	want=$2
+	[ "$got" = "$want" ] || fail "got '$got', expected '$want'"
+}
+
+# drag STARTCOL STARTROW ENDCOL ENDROW
+#
+# Write an SGR Ctrl-mouse press, drag update and release at 1-based positions
+# to the outer pane holding the inner client.
+drag()
+{
+	scol="$1"
+	srow="$2"
+	ecol="$3"
+	erow="$4"
+
+	seq=$(printf '\033[<16;%s;%sM' "$scol" "$srow")
+	$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+	sleep 0.2
+	seq=$(printf '\033[<48;%s;%sM' "$ecol" "$erow")
+	$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+	sleep 0.2
+	seq=$(printf '\033[<16;%s;%sm' "$ecol" "$erow")
+	$TMUX2 send-keys -t "$OUTER" -l "$seq" 2>/dev/null
+	sleep 1
+}
+
+cleanup
+
+$TMUX new-session -d -s inner -x 80 -y 24 || exit 1
+$TMUX set -g mouse on
+$TMUX set -g default-command 'sleep 100'
+
+$TMUX2 new-session -d -x 80 -y 24 "$TMUX attach -t inner" || exit 1
+sleep 1
+OUTER=$($TMUX2 list-panes -F '#{pane_id}' | head -1)
+[ -n "$OUTER" ] || fail "No outer pane."
+BASE=$($TMUX list-panes -F '#{pane_id}' | head -1)
+[ -n "$BASE" ] || fail "No base pane."
+
+# Drag from SGR column 3,row 1 to column 10,row 5. These are window positions
+# 2,0 to 9,4. With the default border, the floating pane content is inset by
+# one cell and has size 6x3.
+drag 3 1 10 5
+
+id=$($TMUX list-panes -F '#{?pane_floating_flag,#{pane_id},}' | tail -n 1)
+[ -n "$id" ] || fail "no floating pane created"
+
+must_equal "$($TMUX display-message -p -t "$id" '#{pane_left}')" 3
+must_equal "$($TMUX display-message -p -t "$id" '#{pane_top}')" 1
+must_equal "$($TMUX display-message -p -t "$id" '#{pane_width}')" 6
+must_equal "$($TMUX display-message -p -t "$id" '#{pane_height}')" 3
+
+$TMUX new-pane -d -M -L -t "$BASE" 'sleep 100' || fail "new-pane -M -L failed"
+
+cleanup
+exit 0