Use the attached project to reproduce.
Workaround:
private void RadForm1_SizeChanged(object sender, EventArgs e)
{
    toolWindow1.EnsureVisible();
}
		Thank you for your input Mieke.
Global solution, addhandler to the Form_Sizechange ->
And use this :
  For Each Obj As Telerik.WinControls.UI.Docking.DockWindow In Sender.RadDock1.DockWindows
                            Obj.EnsureVisible()
                        Next