When I set DF to vertically split a monitor it doesn't seem to do anything either. Apps startup as though they are just ignoring the DF windows. I have three monitors running off a GeForce GTX760 and I was trying to split the middle monitor to see how it works. I may later get a single 4K monitor and split it, if I can figure out how this DF feature works.

I just noticed that some apps will maximize to the DF window, but games inside Steam completely ignore the DF windows, and don't even recognize them as a resolution option. In order to make a 4K monitor work the way I want, I need to have full screen and game apps also recognize the DF windows.