From Xojo Documentation
New in 2021r3
Used to add a separator to a window.
If the width is greater than the height, the separator is horizontal; otherwise it is vertical. In either case, the smaller value controls the thickness of the DesktopSeparator, not the thickness of the visible line in the center of the control. Mouse movement events are detected over the entire area of the DesktopSeparator, not just the visible line.