So ok, I've found a way to get it working at least to the point where I can use both displays comfortably on Windows. This is only for NVIDIA graphics cards, but I'm sure there's a way to achieve...

Well, damn, that sucks... and btw, if Microsoft was waiting on Apple to get it workin, well... it looks like they already have, because on the same PC, OS X 10.10 (hackintosh) works like a charm in...

I'm experiencing the same issue as pxl8 and Belisarius with my new Dell P2715Q (4K UHD) and a 1920x1200 display on the side. When I set the 4K display as my main monitor, everything in the side...