Open Radar

 
Summary:
Since installing the latest seed, when I unlock my screen, my external monitor (a Dell P2415Q) is not detected by the Mac, and the Mac is not seen by the monitor.

Steps to Reproduce:
1. Connect a Dell P2415Q monitor using the mini-DisplayPort to DisplayPort cable that came with the monitor.
2. Use. Enjoy.
3. Lock screen (with Control-Shift-Eject) and go away for a couple of minutes (e.g., to get a glass of water).
4. Return.  Tap Shift to get a login screen.

Expected Results:
Both monitors should light up and show the desktops assigned to them.

Actual Results:
iMac screen lights up, allows log in.  External monitor comes on (wakes), stays black, then displays a message about going to sleep, screen goes black again, power light flashes slowly.


Version:
OS X 10.11.2 (15C47a)

Notes:
If I put the machine to sleep (Apple->Sleep) and wait a little bit before waking it back up, the external monitor is seen by the Mac and vice versa.

As I lock and unlock my screen many times a day, this behavior is really annoying.

Configuration:
iMac15,1
OS X 10.11.2 (15C47a)
Serial Number C02P300WFY6F

external Dell P2415Q monitor, 2304 x 1296 @ 60 Hz, connected over DisplayPort (Thunderbolt port on Mac to full-size DisplayPort on the monitor).