Issue about the external display on macOS Mojave After I install macOS Mojave, the external display became very strange.

First of all, I have a MacBook Pro (15-inch, 2016), and I'm very sure I'm not in the mirror mode because I can put different applications on each display.


When I tried to switch between desktops on the external display (that's my main screen), the one on the MacBook will switch with it. This is very annoying because the desktop I can use now is very limited.
When I play the movie in the full-screen mode, the other display will be pure black.


These only happen after my update to Mojave, so I guess it is the problem of the system. Am I the only one have this problem?
Best guess, you have unchecked 'Displays have separate Spaces' in System prefs> Mission Control...
or, alternatively, you don't have more Spaces than screens, so they are forced to share.