I'm not sure if this is a hardware, software, or OS issue... I have a 15" rMBP (mid-2015) with El Capitan. With the lid shut, and the laptop asleep, I plug it into my two displays (same Dell 24" displays), one via HDMI and one via miniDP. Lately, when I do this, one of the displays shows a fractured image of the login screen, with all kinds of artifacts, lines, and black areas ... then the beachball ... and then nothing. I have to open the lid, shut it down, and boot it up again. When I do that, so long as one of the display cables is plugged in, it lets me login as normal. Any ideas?