One of the big additions in OS X Mavericks, the new version of OS X due out this fall, is much richer support for Macs with more than one display. Macs have supported multiple displays for ages, but only with Mavericks do all the workspace-ordering features of Mission Control, including Spaces and full-screen mode, truly take advantage of a second display. (And now any TV connected to an Apple TV can be an extra display, too.)
For the last year or so, I've been using my MacBook Air at my desk with the lid closed. But I've spent the last few days running Mavericks full-time at my desk on a MacBook Pro (a review unit pre-loaded with Mavericks by Apple) attached to an external display. Mavericks will make using two displays--especially a smaller display the like one you've got on a laptop with an external monitor--a pleasure.
Two screens, two sets of spaces
OS X Lion added support for full-screen apps and coalesced all of its window-management features into one place, Mission Control. These features were nice for people with one screen, but users who work with multiple displays have felt left out. If you popped an app into full-screen mode on one monitor, the other monitor went blank, displaying only a stock linen pattern as dark as my heart. All spaces encompassed both displays, too, so when you switched between then, the content on both displays changed.
What Mavericks will provide is more or less exactly what a multiple-monitor user like me was pining for: Two screens that act independently, each with its own spaces and its own full-screen mode. When I drag the Calendar app onto my laptop screen and click the full-screen icon in the top-right corner of its title bar, it expands to fill that screen. Meanwhile, my larger external display remains fully functional.
Each screen can have its own collection of full-screen apps and its own sets of desktops. When the Calendar app is displaying in full-screen mode on my laptop screen, I can move my cursor to that screen and swipe with three fingers to switch to other views. I can switch to a desktop view, another full-screen app, or even the Dashboard. (Yes, Dashboard still exists! It's gotten only a few new abilities, including a new sparkly effect when you add a new widget and the ability, for the first time, to move it from its leftmost space.) As I swipe from screen to screen on my laptop, the external display remains blissfully still, showing me all my other stuff. As it should be.
If you really prefer the old arrangement, you can still choose to have spaces remain constant across your displays, thanks to an option in the Mission Control preference pane.
No monitor? No problem.
Not everyone has the luxury of owning a display to pair with their laptop or iMac. But if you've got a TV and an Apple TV, with Mavericks you can still have an external Mac display. While Mountain Lion introduced AirPlay Mirroring--the ability to display the contents of your Mac's screen on an HDTV connected via Apple TV--Mavericks lets you just treat that TV as a full-on second display.
Making the connection isn't much different from how AirPlay Mirroring works in Mountain Lion. If you're on a local network containing Apple TVs, an AirPlay icon shows up in the menu bar. You can select an Apple TV from the menu, and choose to mirror your current display or extend the desktop. (If you're running both an external display and an AirPlay display, you can also opt to have the same screen mirrored on all three of them, or have the AirPlay display mirror either display.)
Running in extended-desktop mode, the TV becomes just another display. You can set an arrangement via the Displays preference pane, use Mission Control to manage spaces and full-screen apps, and all the rest. (Apple says that AirPlay requires a second-generation Apple TV or later and a 2011-era Mac or later.)
The display being driven by Apple TV does suffer a little bit from lag. As I moved my finger on my MacBook Pro's trackpad, the cursor responded, but it was definitely a little bit behind where I expected it to be. As a result, I moved the cursor much more carefully on the TV display and was careful not to overshoot and click in the wrong place.
The lag was much less than I expected, and I'd consider it usable, but it's noticeable. It was reminiscent of the lag I've experienced when I've used Air Display to turn my iPad into a second small external display. In fact, I was able to use Air Server to turn my MacBook Air into a second display, with similar results. If you've got an old iMac around, you may be able to use this approach to turn it into a serviceable second display.
So far as I can tell, you can only use one Apple TV at a time as an external display, and you've got to have a "real" display connected as well.
One dock, two menu bars
Up to now, the OS X menu bar was inviolate: There was only ever one menu bar. With Mavericks, though, each display can have its own menu bar. As I type this, I'm looking at a BBEdit menu bar on my laptop display and a Maps menu bar on an HDTV across the room. The currently active app's menu bar looks like you'd expect it to look; the inactive app's menu bar is much less opaque. When you switch displays or click on the currently inactive menu bar, their transparency (or lack thereof) swaps.
Separate menu bars for each display may seem obvious, but it will be a productivity booster for users of multiple displays. Before now, even if you used a second monitor on your Mac, the menu bar would remain on the primary display, necessitating a lot of mousing back and forth unless you installed a quirky add-on menu utility like SecondBar.
So you can't turn around without tripping over a menu bar in Mavericks. Fans of OS X's Dock will wonder if the same is true for the Big Shelf O' Icons. The answer is no: There's still only one Dock, ever. If you set your Dock to display on the left or the right, it will appear on the leftmost (or rightmost) display and stay there forever.
If you set the Dock to display on the bottom, however, something strange happens. The Dock follows you around, sort of. When I move my cursor to an external display and start working in an app over there, the Dock remains where it was, on the first display. However, if I move my cursor to the bottom of the display (as if trying to summon a hidden Dock), the Dock slides out of view on my first display and slides back into view on the second. If your Dock is set to auto-hide, it may end up seeming seamless.
(In the beta version of Mavericks I'm testing, the Dock sometimes gets stuck on one screen or is reluctant to move, but I suspect this is just a beta bug, not an intended behavior--we'll see what happens when the final release arrives this fall.)
A few other long-time Mac windowing conventions have changed with the introduction of these features. Most notable is the ability to place a window so that it straddles two displays; in Mavericks, as you drag a window from one screen to the other, it begins to fade away--and then reappears on the other display. No halvsies.
O happy day
Multi-display Macs have always been with us. The first Mac I used was an SE with an external portrait-orientation display for page layout. At my first job, my colleague Rik Myslewski was a die-hard multiple-monitor user. Studies have even shown that multiple monitors increase productivity.
Mac OS has always tolerated users with more than one display, but it's never truly embraced them--until Mavericks. I honestly never thought I'd see the day that Apple added a bunch of features targeted at the multiple-monitor crowd, but that day is finally coming with the impending release of Mavericks.