From Xojo Documentation
The distance (in pixels) between the top edge of the screen and the top edge of the content region of the window.
The Top value refers to the topmost part of the window content region, which is the area starting below the Window Title Bar.
Setting Top = 0 moves the top of the windows content area to the top of the screen. On macOS, this puts the window behind the main menu bar that appears at the top of the screen. To position a window directly below the main menu bar, you should use Screen.AvailableTop to get the topmost position after the menu bar and then use Window.Bounds to position the window.