That's what I'm wondering. The error we are getting on alt-tab is related to ffxivgame.exe losing DirectX device, which is normal behavior that has to be handled as explained
here.
Now, you might say that it's impossible with the current engine.
But that isn't true; remember the first windower that was released not long after launch of FFXIV? The one SE shut down for hacking itself into the exe?
Well guess what, that little program fixed the crash on fullscreen and allowed you to run Crossfire/SLI
without even having access to the source code.
So my question is, do you really want to tell me SE can't implement the functionality "because of engine limitations"?