P2418D as second, moves Windows to main after waking

Always include exact PC model and version of Windows in your posts.

How is the 2nd monitor connected to the PC: HDMI, DP, DVI...?

And importantly, are both monitors connected to the same add-in video card, or one to an add-in video card and one to onboard Intel HD Graphics, and which monitor is connected where?

Sorry for the lack of detail.

It's a rig I built with Windows 10 64 bit up-to-date, running on an Asus X99-S motherboard with i7-5820K CPU.

The GPU is an Asus nVidia 1070 (TURBO-GTX1070-8G).

Both monitors are are connected via Display Port. The Dell monitor with the problem is the secondary display.

For now I switched display sleep off and am just using the screen saver.

I narrowed it down to the GPU dropping the signal when Windows makes the screens go into standby. The main monitor works as expected, but the Dell reports it can't find a signal. Upon waking the window is there shortly, but it seems like the Dell resets and the window gets shifted to the main.

So both monitors are connected to DP on the Asus NVidia GPU..?

Since they're both connected via DP, what happens if you swap the DP port connections from the GPU to the 2 monitors, then reboot and tell Win 10 which is monitor #1 and which is #2 now?

If that doesn't help, you can try changing the options on the Win 10 Power options screen:

1. Open Power options screen and identify the active power plan
2. Click the link to change its settings
3. On next screen, click link for Additional Options
4. On that screen, disable Hibernation, disable Hybrid Sleep, and disable PCI-Express Link State Power Management. And disable USB Selective Suspend for good measure.
5. Save the changes and reboot...
6. Does that help?

And if that still doesn't help, maybe it's time to contact Asus tech support...