From Xojo Documentation
Supported PlatformsProject Types: Desktop Platforms: macOS
When set to True, the window control to enable macOS full screen mode is displayed.
This property can only be changed at design-time.
Clicking the control enables the macOS native full screen mode. This property has no effect on Windows or Linux.
In order to test if the user is running in the native full screen mode you should check both full screen properties:
When FullScreenButton is False, setting Window.FullScreen to True cause the Window to fill the screen although not using the native full screen mode so the behavior will be different.
Starting with macOS 10.11 (El Capitan), the Maximize button is treated as a full screen button. You do not have to specifically set FullScreenButton to True. In order for the user to maximize a window rather than put it into Full Screen Mode, they should Option-Click on the Maximize button or double-click the title bar.