I am running OS X El Capitan (recently it seemed to come out of beta, although I've been having this problem throughout the beta so it shouldn't matter). Split screen works completely fine in all regards except one. If I put messages in a split screen with another app it hogs the screen. http://imgur.com/b2pkpAc.png If you look at the "mail and messages" screen, you can see that only messages appears (and if I go to that space it is as if I only have messages maximized). If I open mail manually, it jumps to the space and looks like this: http://imgur.com/FG7kBHt.png The divider is gone, and if I click on messages it again hogs the whole screen. To reiterate, this only happens with the messages app.