External monitor moves windows when coming out of sleep (2012 rMBP)

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?

It may be actually worth resetting the NVRAM and SMC (in that order) to see if this resolves your issues. Before doing so, unplug all external devices (including monitors, keyboard, etc).

Resetting NVRAM on your model MacBook Pro

Your Mac use Non-Volatile Random-Access Memory (NVRAM) to store a whole range of settings. Here’s how to reset this:

1. Shut down your Mac. Yes, a full shut down, not just logging out.
2. Press the power button and then press the commandoptionPR keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
3. Hold those keys down until your Mac reboots again and you here the startup chime for the second time.
4. Let go of the keys and let your Mac reboot normally.

Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Now proceed to reset the SMC.

Resetting the SMC on your model MacBook Pro

The System Management Controller (SMC) impacts a whole range of power management functions. Here's how to reset this:

1. Shut down your computer
2. Disconnect the MagSafe plug (power cable) from the computer
3. Remove the battery
4. Press the power button for 5 seconds and release
5. Put the battery back in
6. Reconnect the MagSafe cord (or power cable)
7. Turn your computer back on with the power button