From Xojo Documentation
Used to get information about the monitors connected to the user’s computer.
The following reports on the values of AvailableLeft, AvailableHeight, AvailableTop, and AvailableWidth for the main screen. The value of AvailableHeight takes into account the height of the taskbar (Windows and Linux) and the Dock (Macintosh). If the user has these tools configured to display on the side, the AvailableWidth property takes them into account.
s = "Left=" + Str(Screen(0).AvailableLeft) + EndOfLine
s = s + "Width=" + Str(Screen(0).AvailableWidth) + EndOfLine
s = s + "Top=" + Str(Screen(0).AvailableTop) + EndOfLine
s = s + "Height=" + Str(Screen(0).AvailableHeight) + EndOfLine