I have two monitors on my Mac. I have just installed Maverick and I notice I can no longer stretch a window from the screen on my left to the screen on my right. Mac OSX Maverick 10.9 seems to pick one screen and it does not show the stretched window on the other screen anymore.
This is a problem because I like to use one particular program (VMWareFusion) to do my software development in full screen mode. No longer can I see my guest operating system in both screens anymore when I full screen it because of this problem... at least that is my hypothesis.