ListBox.SelectionRequired
From Xojo Documentation
Property (As Boolean )
aListBox.SelectionRequired = newBooleanValue
or
BooleanValue = aListBox.SelectionRequired
New in 2019r2
Supported for all project types and targets.
or
BooleanValue = aListBox.SelectionRequired
New in 2019r2
Supported for all project types and targets.
If True, users will not be able to deselect the last row by clicking below the last visible row or by dragging. You can still deselect the last row by setting the SelectedRowIndex property to -1.