In El Capitan, Mail prioritizes showing you new mail in your Inbox and prioritizes the downloading of the messages or mailboxes that you're currently viewing. It's an improvement that's been a long time coming thanks, slow Virgin America in-flight Wi-Fi! but it's finally here.
Mission Control and split-screen view
Only Apple knows for sure, but it seems that Mission Control is used largely by power users and not your average everyday Mac user. Still, new ways of moving and displaying windows find their way into every OS X release, so clearly someone at Apple still loves them. In El Capitan, Mission Control's been revamped, with every window getting its own thumbnail previously all windows from an App would be stacked together. All thumbnails are distributed while keeping geography in mind, so a window that's in the bottom-left corner of the screen would have its thumbnail appear in the bottom-left corner of the Mission Control view.
El Capitan's major new feature in this category, though, is the split-screen interface. This feature echoes the new split-screen app view in iOS 9, and it adds an extra dimension of utility to full-screen view. If you've got a large monitor, full-screen view is often overkill, because few apps really need to take up the entire screen. With El Capitan, you've got the option of having two different apps share full-screen view.
Entering split-screen view is actually fun, and quite clever on Apple's part. If you click and hold on the green plus/maximize button in a window's title bar, you'll be prompted to choose which side of the screen you'd like that window to be placed on. Then Mission Control will display on the other side of the screen, letting you choose a currently open window to use as the first window's split-screen buddy. It's fast, cool looking, and efficient.
Converting full-screen mode to split-screen mode isn't without its interface quirks. I noticed that, depending on how an app presents itself in full-screen mode, sometimes it could be very hard to tell which app was active/frontmost. That sometimes led to unexpected behavior for example, I tried to zoom in on a PDF in Preview by spreading my thumb and index finger on the trackpad, but it didn't work because I hadn't clicked on the window to activate Preview yet.
Since it seems that split-screen view is just a modified version of the old full-screen view, there are probably going to be some quirks like this with apps assuming they're the only app you can see because you're in full-screen mode, even though they're not until apps get modified to adapt to the El Capitan world.
(Speaking of apps getting full-screen/split-screen updates, Mail is a good example of this. In El Capitan it's got several new full-screen features, including the ability to minimize composition windows and even add multiple composition windows in a tabbed interface.)
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