I don't have another monitor with a DisplayPort connection to try. My monitor supports DisplayPort, DVI, and HDMI connections. When I connected with DVI, Windows is able to "power off" the display and actually powering off and on the display didn't result in the device disconnected/connected events and resizing of windows. With the display connected via HDMI, Windows was NOT able to power off the display (just goes black but not to low-power state) but I didn't get the device disconnected/connected events and resizing of windows when I did power off the display. I don't have any video adapter installed in the system to test any other connection methods. When connecting with DVI my resolution is only 1920x1080 (DH87MC doesn't support dual-link DVI) and with HDMI I also only got 1920x1080 max resolution. (I'm not sure if that's a motherboard/processor issue or a display limitation -- I thought the display was supposed to support the full 2560x1440 with either DisplayPort, dual-link DVI, and HDMI.) I also tried DisplayPort with the resolution dropped to 1920x1080, but that didn't make any difference.
I had already updated to the latest BIOS level. I might be able to swap the power supply from my old PC, but I haven't tried that yet.