Skewed Log In Screen When Screen Sharing

Trying to use a Mac Pro as a headless media server. Worked just fine when I had a monitor or TV hooked up to it, but now that I've tried to remove a monitor and use it headless, I get a skewed log-in screen and an unusable operating system.

Here's what it looks like when I screen share from my rMBP (though the problem is not unique to my rMBP, I also get the same look from VNC on my iPad, an older iMac, and a non-retina Macbook Pro):

I've tried resetting my com.apple.windowserver.plist, among other ideas. Screensharing returns to normal function if I plug in a monitor for even a short moment. Somehow I am getting a problem with the default resolution when using the Mac Pro without a screen.

Any ideas would be greatly appreciated, thank you.

Reconnect the monitor. On the Mac Pro, set the resolution to a lower setting (as a test to the lowest possible one). Then try again with the monitor disconnected.

If it works, you can then increase the resolution.

I've had similar problems happen several times over the years. Sometimes connecting a display adapter to the video card (even though it is not connected to anything) will fool the video hardware into working properly.