From Xojo Documentation

Read-Only Property (As Integer )
IntegerValue = aSystem.MouseY

Supported on Desktop.

The Y coordinate of the mouse (points). Measured from the top-left corner of the screen.


This example gives the X and Y coordinates of the mouse when an event fires. For example, it works in a MouseDown or MouseEnter event.

MessageBox("X=" + System.MouseX.ToString + " Y=" + System.MouseY.ToString)