A projector must be an extended (not mirrored) display before launching projection software

Before running a realtime projection tool the operating system must treat the projector as an independent display, not a mirror of the laptop screen — otherwise the interface and the projected output end up on the same screen. On macOS, uncheck ‘Mirror Displays’ in System Preferences > Displays, and on 10.11+ also turn off ‘Displays have separate Spaces’ in Mission Control to prevent a menubar appearing in fullscreen. On Windows, use Windows+P and choose ‘Extend’. Only then do you drag the projection software’s output window onto the projector display before going fullscreen; if you go fullscreen first, the output fills the interface display instead of the projector.

Examples

Mac: System Preferences > Displays > uncheck Mirror Displays. Then drag the VPT output window onto the projector and press Esc for fullscreen.

Assessment

Walk through the macOS steps needed before launching a projection tool so the output lands on the projector, not the laptop. What happens if you go fullscreen before moving the output window?