When additional displays are connected, macOS uses either extended desktop or video mirroring (not to be confused with text flipping). Extended desktop is two or more displays being used as additional screen space (usually the default). Video mirroring makes another screen look exactly the same as the main screen. You will want to use extended desktop if you need to use text flipping and general dual-screen use. You may want to use video mirroring for performance reasons if you do not need to use text flipping.
To change extended desktop or video mirroring:
- Connect a second display, if necessary.
- Open System Preferences and select “Displays”.
- Select the “Arrangement” tab. To use video mirroring turn on “Mirror Displays” (both displays need to be at the same resolution for it to work). To use extended desktop (if video mirroring is on) turn off “Mirror Displays”. On some systems pressing ⌘F1 or F7 toggles video mirroring.