I recently purchased a Dell P2715Q for the express purpose of using as the primary display for my 2012 rMBP running 10.10.3 when "docked" at home. I have the laptop in a vertical holder that holds it in the closed position. The monitor is connected to the rMBP using a mini display->display port cable, provided by Dell. In case it matters, max res from the laptop is 2560x1440 (my older rMBP apparently can't handle the full 4k res the monitor can do...whatever). My rMBP was the top-of-the-line when I bought it, so it has the NVIDIA GT 650M 1gb.
The problem I am having is that all of the windows get shoved into the upper-left corner ever time I wake the laptop from sleep/screensaver. I am guessing that OS is reverting back to the screen res of the build-in monitor, which has a higher res but smaller desktop real estate, when everything goes to sleep. This is getting annoying as I typically work with many windows open and placed in particular areas. How can I tell OS X to stop moving my windows around?