From Xojo Documentation
If True, the window has an enabled Maximize button in its Title bar.
You can only change this setting while in the Window Layout Editor.
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.