Windows 10 windows all return to main monitor on sleep

The images of the monitors are relative to the pixel resolution. This will be why they all appear the same size regardless or physical dimension of the monitor.

Monitor Resolution

I actually think I found an answer to this. That was surprisingly fast considering the number of dead ends I saw on forums. User nixda on a different question, Turning DisplayPort monitor off disables monitor completely, says:

Disable the "DisplayData Channel Command Interface" (DDC/CI) in your monitor settings.

For my Asus LED monitor this meant going to the monitors settings menu (using the physical button on the monitor) -> System Settings -> OSD Setup -> DDC/CI and turning it off.

Seems to be working so far will update if that changes.

EDIT

I thought the above solved it because when the monitor went to sleep (power light changed from blue to orange) I could quickly wake it up without issue. However if the monitor stayed sleeping longer (say 1 minute+) then it would not wake up. I now don't think the above step is necessary.

I found a post on a dell forum which led me to the solution:

This is a video card, video card driver, or operating system power management issue. The monitor DP (DisplayPort) is passive. It simply waits for the signal from the video card to awake. By powering the monitor off/on, you are forcing the operating system and/or video card to reinitiate the DP handshake. The Radeon HD 7790 has eight power management states through its PowerTune technology. My guess is somewhere in that software is the ability to tell it to adjust what the card is doing as far as power management. (source: http://en.community.dell.com/support-forums/desktop/f/3515/t/19520244 )

Elsewhere in that thread is mention of the TriXX Tweak Utility (direct link) from Sapphire Technology (the maker of my video card). This has a setting Disable ULPS (ULPS = Ultra Low Power State). I installed that utility and checked the Disable ULPS setting and sure enough when my monitor went to sleep I could now wake it back up. Success! Well sort of, when I restarted my computer the problem returned and opening the TriXX Utility I could see that the Disable ULPS checkbox was unchecked. As far as I can tell the TriXX utility has no way to make that setting stick between reboots (I might be wrong about this).

Digging further I found several forums discussing disabling ULPS (mostly in the context of working out crossfire issues, e.g.: How to disable ULPS). In those forums the procedure they recommend is searching the entire registry for EnableUlps and changing the value of each occurrence from 1 to 0 (in fact you will see several minor variations on the exact procedure but this is the gist of it).

For me in particular I needed to change the following keys: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\amdkmdag -> EnableUlps HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\services\amdkmdag -> EnableUlps HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\services\amdkmdag -> EnableUlps

(The EnableUlps setting appeared elsewhere but was already set to 0. Also there's another setting EnableUlps_NA which I did not touch)

If you're like me you'll be reluctant to change the registry to fix a problem that seems like it should have a readily available solution. But as you dig around I think you'll find that ready solutions don't currently exist (and it beats modifying the DisplayPort cable with electrical tape which is an oft suggested solution)

I made the registry tweaks and now it seems to be working properly even after restarting.

Note: people say you will need to redo the registry tweaks every time you update your video drivers.