How to solve an app blocking your screen when your Mac hasn’t crashed
Spaces can solve a foreground app that can’t be dismissed or quit.
After upgrading to macOS Sierra and restarting my Mac, everything came up normally—except for Snapz Pro, a venerable screen-capture program that I had updated to the latest version (2.6.0), but should probably have disabled out of caution.
Snapz Pro launches at startup, but it’s supposed to hide itself without showing a screen. Instead, it had activated and locked one of my two displays. Clicking the red-dot close box on the Snapz Pro window didn’t help, and my second display was dimmed out, as if the capture window were in place. But clicking, pressing Escape, and other increasingly frantic actions did no good. I had a spinning rainbow over that screen, but the other was just fine.
I even tried to bring up a Terminal window to kill the app, hoping that wouldn’t cause other problems, but was unable to get the Terminal window—assigned to my second display—to appear on my main monitor.
That’s when I remembered Spaces. I don’t use it to have multiple workspaces, but I do have the setting in the Mission Control system preference pane for Displays Have Separate Spaces checked. This gives me menu bars in each display and I prefer it to a single virtual display that stretches over multiple monitors.
From the keyboard, I invoked Spaces, and then clicked the + sign to add a new space. I was able to drag the misbehaving app to the new space, reveal my existing spaces, and finally clicking the close box on the Snapz Pro window. Then I deleted the third space, and used the Users & Groups preference pane to select my account, unlock it, click Login Items, and remove Snapz Pro from my list of startup items. I’ll wait to see if there’s a future update, or if I need to use it again.
Colleagues have since told me that they use Spaces routinely to isolate misbehaving apps that stage a screen takeover. It’s a bit kludgy, but it saves you from a forced restart—and facing the same problem on reboot.
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