Dear Aunt TUAW,
Ever since I upgraded to OS X Lion, I've been unable to use dual monitors if I have an application running in full screen on my iMac's display. I would love to be able to fire a movie up on the second display, make it full screen and still make use of my iMac, rather than turning it into a 21.5" television that can do so much more...
Any ways to accomplish this?
Your loving nephew,
Josh, aka Single-Display in Columbus
Lion's inability to properly handle full screen apps across multiple displays is truly an irritating feature, although it's one that Mountain Lion will apparently address.
Instead, she recommends using QuickTime's "Fit to Screen" (Command-3) for that television watching instead of "Enter Full Screen" (Control-Command-F).
Yes, this leaves your desktop wallpaper visible. So if you are really concerned about these things, hop into System Preferences and set your desktop to a solid black (System Preferences > Desktop & Screen Saver > Desktop > Apple > Solid Colors > Black).
Auntie has found one exception to the full screen dilemma: Aperture. According to Uncle Rich, Aperture will use the second monitor when the primary one is in full screen mode. It doesn't do it by default but you can turn it on after entering full screen. This is probably because Aperture's full screen mode predates Lion, though; Aperture has always worked this way.
Want to let Apple know how you feel about dual-full-screen issues? Drop them a note at their bug reporting site.
Apple OS X Mountain Lion
Apple OS X Yosemite