A Mac has a single menu bar at the top of the display that dynamically changes based on the front-most app, and that behavior stays the same, even in the Split Screen view: The app being used takes over the menu, split screen or not. It's an elegant solution for simultaneous use of dual apps.
If you want, the divider can be dragged from side to side to change each app's real estate. This is especially useful for doing research in a browser while, for instance, writing about OS X, and it's something that is already available on Windows. (Apple's version is harder to trigger accidentally, however.)
Before, if you wanted to place an app or any Finder windows in their own virtual Space, you had to launch Mission Control first; now any window that's dragged to the menu bar activates Mission Control, allowing you to drag and drop to create a new Space in full-screen mode, or letting you drag and drop to choose a new or existing Space in which to drop the window. Essentially, Apple reduced the steps needed to perform a pretty common maneuver. If you're a fan of Spaces, you'll appreciate this.
There's a common problem with full-screen app support in Yosemite, especially in apps like Mail: Sometimes you need more than one window open, and full-screen mode didn't allow for that. In Yosemite's full-screen mode, the compose window wouldn't let you back into other mailboxes -- or let you do anything in Mail -- until you finished composing your email. El Capitan fixes this behavior by allowing a compose window to be minimized, letting you back into the app without finishing the message first. And if you're composing more than one message, each one shows up in tabs, just like a browser window would in Safari.
Data detectors in Mail have also been improved, and now offer suggestions at the top of the email body upon finding phrases in a message that could lead to calendar entries, like "Let's go out for dinner at 5:00." Apple marketing calls this suggested events; the same feature is also available for potential contacts.
Finally, Mail offers swipe gesture support, similar to what you see on iPads and iPhones: Using the trackpad, you can swipe left or right in the email list to perform actions like mark as read/unread and delete. If you're accustomed to doing this on an iOS device, the move will come naturally and will help you sort your mail faster.
Safari gets pinned
Safari continues to evolve, and this year gets pinned sites and built-in AirPlay support for videos. (AirPlay allows you to wirelessly beam content to an Apple TV-connected television.) Pinned sites are simple: They're invoked by going to the Window dropdown menu and selecting "pin tab." After you do that, the page will stick to the left of the Tabs bar for quick access in all current and future Safari windows. As for the built-in AirPlay support, it may not be good enough to supplant ClicktoFlash -- that's a Safari extension that does much the same thing and even lets you download content -- but it'll be good enough for users who just want to stream video to Apple TV.
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