With windows 10 there has come some new swap chains, which unfortunately messes some things with regards to fullscreen up.
We've fixed a bunch in the renderer (which would downright cause crashes), but there is one issue remaining, which seems to be related to slate.
Repro is simple:
Start any level with -game
type 'r.setres 800x600f' in the console.
press the windows key to return to windows.
Observe the window, which is no longer in focus, is now the full size of the screen, but still renders at 800x600, in the corner of the window
I tried to fix it: I think if you're in exclusive fullscreen, the correct behaviour is to not render anything(show a blank screen) - However, Slate switches to windowed mode as soon as you tab out.
The engine current correctly switches back to exclusive fullscreen.
Please don't hesitate to write me, if you need help or want to talk about the issue!
We have 2 or 3 users contacting us via udn, so hopefully its something you can look at it sometime soon.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-92447 in the post.