When connecting a projector to a Mac, the default behaviour is to mirror the contents of the Mac's built-in display on the projector's display.

There appears to be no built-in setting (visible in System Preferences or a hidden setting) to change this default behaviour.

An option to change this default behaviour to allow for attached projectors to be used as extended displays by default would be most helpful.

Note that presenters typically visit many venues, and so are constantly connecting to projectors that are "new" to their Mac.