I just bought a P2415Q Monitor and connected it to my 2015 Macbook Pro with the DisplayPort to thunderbolt cable that was included with the monitor. It seemed ok at first, but after the first time the computer went into sleep mode I noticed the monitor wouldn't wake up. The message on the monitor was:
There is no signal coming from your device. Please activate your device to wake it up. If there is no display, press the monitor button now to select the correct input source on the On-Screen-Display menu.
There is some information about this message in the monitor's User's Guide, but nothing that helps.
Through a combination of just random experimentation (unplugging/plugging, turning off/on, etc) I was eventually able to get the monitor and laptop connected again, but the problem recurred the next time the computer went into sleep.
I've now also seen a bunch of posts online complaining about similar issues with this monitor or the 27 inch variation. Does anyone have any suggestions about resolving this? I was thinking about getting an HDMI to HDMI cable and seeing if that was an improvement over the DP-mDP connection.
Another thing I notice while experimenting with connections was that when I plugged the DP cable into the monitor's DP-OUT connector it seemed to work just as well as plugging it to the DP-IN slot (i.e. worked initially but failed to wake up from sleep). Is that normal that the OUT would also work as an input?