From Xojo Documentation
When set to True, the window resizes itself to cover the entire screen.
The values of the Left, Top, Width, and Height properties are adjusted to reflect the window’s new position. They cannot be changed as long as FullScreen is True. If MenuBarVisible is set to True, the top of the window will be partially covered by the menu bar in macOS. In the case of multiple monitors, the window resizes to fit the monitor which contains the greatest portion of the window.
FullScreen defaults to False.
On macOS, you can instead set Window.FullScreenButton to True to let the user enable the macOS full screen mode.