Window.Persistent (gb.qt4)

Property Persistent As Boolean

Indicates if the window is persistent, i.e. if it is not destroyed but only hidden when the users closes it.

GUI applications automatically quit when the only visible window is closed whether or not it is set Persistent.

You would set the Persistent property when you don't want to recreate your window each time you use it; usually when it's a dialog, or a tool window.