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.
But as I wrote last October, 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.
Check Displays Have Separate Spaces to avoid the problem of other displays going blank when in full-screen mode on your primary display.
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.
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Author: Glenn Fleishman, Senior Contributor
Glenn Fleishman’s most recent books include Take Control of iOS and iPadOS Privacy and Security, Take Control of Calendar and Reminders, and Take Control of Securing Your Mac. In his spare time, he writes about printing and type history. He’s a senior contributor to Macworld, where he writes Mac 911.