Q: Dual monitors and fullscreen fiasco, is there a work around?

Found the answer here:

https://www.reddit.com/r/osx/comments/2kkwb9/yosemite_multi_monitor_fullscreen_m ode_all_other/

[–]Troll__McLure 10 points 1 year ago

I have the 'displays have separate spaces' enabled but it still does not work. Second monitor is just a blank, gray screen when going fullscreen.

I've been trying to figure this out for so long. I fixed the problem found from another site.

I also work on my main screen and want to watch movies as background.

Copied from another link:

OS X's full-screen mode has had gradual improvements since its introduction a few releases ago. In Yosemite, it became more deeply entwined with Spaces and a primary “zoom” method: click the green button in a window by itself, and the app or window switches to full-screen mode. (Hold down Option to revert to the old action, which is to zoom the window to fit the display better.)

Until Mavericks, full-screen mode worked with only a single display on a Mac with other monitors, besides other quirks. Preston Hiers seems to be encountering one of these:

Going into “full-screen” mode on my apps causes my second display to go black. It just started doing this today after I did a restart. I am using a new 27-inch iMac running El Capitan. I have had my computer for over a month and never had this problem before.

It’s related to Spaces, which has been a thorn in my side for years. Spaces lets you create multiple desktops and that includes an option to set each display to be a separate space. You have to have that box checked in the Mission Control system preference pane to use apps full screen on different monitors at the same time.

Spaces is a finicky beast. I was long unable to get this mode to let me stick apps on my office Mac with two displays to my preferred monitor. El Capitan fixed this for a while, but then I’m back to where I am. When I launch apps any given morning, they appear arbitrarily on my first display or second, regardless of where I assigned them.

From what I can tell from others’ troubleshooting of this blank-second-monitor issue Preston is having, it’s tied to the same feature. You can try unchecking the Displays Have Separate Spaces box, then logging out ( > Log Out account name), log back in, check the box, log out, and back in. This might clear the state. Rebooting might help, too. But it’s clearly a bug—you didn’t trigger a feature unintentionally.

I just posted a link below but see if this helps. Worked for me once I changed the settings in

Preferences>MISSION CONTROL>

Than log out or reboot computer. Boom should now have your second screen working with no blackout.

http://www.macworld.com/article/3056641/macs/how-to-avoid-a-blank-second-display -when-using-full-screen-mode-in-an-os-x-app.html