src/Pure/GUI/popup.scala

   layered: JLayeredPane,
   component: JComponent,
   location: Point,
   size: Dimension)
 {
-  def show
+  def show: Unit =
   {
     component.setLocation(location)
     component.setSize(size)
     component.setPreferredSize(size)
     component.setOpaque(true)
     layered.add(component, JLayeredPane.DEFAULT_LAYER)
     layered.moveToFront(component)
     layered.repaint(component.getBounds())
   }
 
-  def hide
+  def hide: Unit =
   {
     val bounds = component.getBounds()
     layered.remove(component)
     layered.repaint(bounds)
   }