From Xojo Documentation

Read-Only Property (As Integer )
IntegerValue = aDesktopDisplay.Top

New in 2021r3

Supported for all project types and targets.

The top coordinate of the display relative to the main display. For the main display, it is zero. Use the DesktopDisplay.DisplayAt method to call Top. For example, DesktopDisplay.DisplayAt(0).Top returns the top coordinate of the main display.