OS X: Suddenly only one window/application visible at the same time

I have a rather weird problem with my desktop of my MacBook Pro. I use El Capitan 10.11.15.

For reasons I don't know I'm no longer able to keep more than one running application visible at the time.

For example, if I start Terminal, it opens. But when I start TextEdit, Terminal will hide or minimize and TexEdit will become visible instead. The same goes for every other application.

When I open Mission Control, it also shows only one active window, no matter how many other applications are running.

I must have accidently activated it but cannot recall how. I've tried playing around with Dock and Mission Control settings, but nothing changed it.

Does anyone know how to get it back to normal so that I have more than one window/application visible at the same time?

Solution 1:

This answer lifted in its entirety from Super User : Snow Leopard desktop - I want two files from two different applications open at the same time. Won't let me

Have you per chance turned on "Single App Mode"? This hidden preference causes Mac OS X to hide all applications but the current active one.

You can tell by running the following command in Terminal.app:

defaults read com.apple.dock single-app

If it prints a 0then it is not turned on. If it prints a 1 then yes, it is turned on.

You can disable this trick by running:

defaults write com.apple.dock single-app 0

Solution 2:

After fiddling around for hours, I found out what had caused this bug. It was one of the autostart elements. I don't know exactly which one it was, but after removing all and only adding what I really needed, all worked fine.