So after the several headaches of the updates, here would be the last issue I can think of that came as a result of the 8.1 update.

I have it for my video players and my Steam games. (screencap attached)

Once the window opens, I have to either drag it manually, use the Wni+Shift+[left or right] combo, or use the tittle bar button to switch monitor.
Those still work, though a fullscreen video player won't budge. It has to be windowed to do so.

Are there any other details you'll need to narrow down the source of the issue?

I have no idea what changed but things started working again.

I changed the settings in the media players to have themselves go into my second display, without DF needing to intervene.

I tested a Steam game this morning with the Logs enabled and, well, it worked!

-Recent changes
--Yesterday I got a driver update for my Nvidia GPU(s)
--I've been constantly trying to fix the issue of Windows 8.1 automatically messing with my DPI settings on my larger display (no luck so far)
--I enabled the Auto-Hide of the DF Taskbar

Honestly, I don't know what fixed it and I still have that 8.1 issue to deal with. But well, just thought I'd give you the good news.