Supposedly there’s a setting to select extend/mirror mode but I can’t find it in ANY settings client or server. It simply doesn’t exist.
The Windows screen is simply mirrored which is quite frankly, pointless.
Screen duplication and extension can be changed in the server device’s Display Settings menu than can be accessed by right-clicking on an empty part of the desktop and selecting Display Settings on the menu that appears. For further information on desktop extension, you may check our User Manual on the section “Configuration and Settings” -> “Desktop Extension or Desktop Duplication (mirror) Settings in Display Control Panel” https://forum.spacedesk.netuser-manual/#item_470
Let us know if you have other concerns and we’ll be happy to help.