Update: I set the power management to "do nothing" when laptop lid is closed on AC power. Even so, the LCD (or at least the back-light) does extinguish when the lid is closed, and the external remains on as desired.
Update: I set the power management to "do nothing" when laptop lid is closed on AC power. Even so, the LCD (or at least the back-light) does extinguish when the lid is closed, and the external remains on as desired.