Except in El Capitan, when suddenly Full Screen Mode is now capable of running two apps at once.
When I was writing my first look at the El Capitan public beta, I used Split View to read Apple's PDF reviewers guide in Preview while taking notes in Notes. Neither app displayed a traditional window title bar or toolbar unless I moved my cursor to the top of the screen. As a result, the only way I could tell that Notes was the active application was when I would see an insertion cursor blinking in my note.
Apps behave differently when they're active and not, a distinction that becomes more important in Split View. When I was taking notes in Notes, I was also scrolling through the PDF in Preview. Mac apps are happy to let you scroll through their content when they're not active--just move your cursor over the content you'd like to scroll, put two fingers on your trackpad, and there it goes.
However, other behaviors aren't allowed when an app isn't active. If I wanted to zoom in on something in that PDF I was scrolling through, I couldn't do it unless I clicked somewhere first. That click would make Preview active--though nothing on my screen would indicate that other than the fact that the insertion cursor in my Notes document would disappear--and then I could zoom and scroll.
The version of Split View coming in iOS 9 doesn't suffer this problem, mostly because there's no abstract pointing device on iOS. When you're touching a window to zoom in, you're touching it--it becomes active immediately. But when I move my Mac cursor over a window in an app that's not active, I can scroll (but not zoom) without ever "touching" it by clicking. Slightly different metaphor, and a very different experience.
I'm not sure what the solution is here--or if there is one. Since you can scroll content even when an app isn't front most, maybe Apple needs to extend that functionality to cover other gestures. Maybe apps that support Full Screen Mode--including Apple's--need to be modified to deal with the fact that they may now be sharing space with other apps.
I still believe that Split View is going to be a productivity benefit to many Mac users. But it's interesting to see how adding one new wrinkle to a longstanding OS feature can lead to some unintended and weird consequences. Here's hoping most of these wrinkles are ironed out before El Capitan arrives in the fall.
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