Oh Apple…
Since…oh, I don't know…the last two O/S updates I've been having issues with my external monitor coming on when the MacBook comes out of sleep mode. It's not that it doesn't turn on; it's that the displays are for some reason swapped from what they had been. In other words, the external monitor that was being designated my main display shows what had been on the laptop screen and vice-versa. The only way to get it to (quickly) reset itself is to unplug it from the laptop and plug it back in.
Well yesterday I needed to set the external monitor to its native resolution in order to do a screen capture of a Dell quote for work (don't ask). Anyhow the thought came to me to leave the monitor at the default resolution (I normally set it a bit larger—or is that smaller?—for my aging eyes) and see if everything works normally when it comes out of sleep.
Voila! Woke the machine up this morning and everything displayed as it should.
Stupid Apple.
"It just works!"