I work on a Mac with a two-monitor setup and one day I was working on an online course. I tried to watch a video on my secondary monitor (a laptop) and work on the primary screen (a 27" display). The video was a bit small, so I toggled it fullscreen on my secondary monitor. The video indeed became fullscreen, but at the same time my primary screen went black.
This behavior, however, can be changed in macOS preferences. Go to
System Preferences >
Mission Control and toggle
Displays have separate Spaces.
Next time you toggle an app full screen, the other screen should stay usable.
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