src/Tools/Graphview/graphview.scala

     x1 = (x1 + metrics.gap).ceil
     y1 = (y1 + metrics.gap).ceil
     new Rectangle2D.Double(x0, y0, x1 - x0, y1 - y0)
   }
 
-  def update_layout()
+  def update_layout(): Unit =
   {
     val metrics = Metrics(make_font())
     val visible_graph = model.make_visible_graph()
 
     def node_text(node: Graph_Display.Node, content: XML.Body): String =

       curr = (curr + 1) % filter_colors.length
       filter_colors(curr)
     }
   }
 
-  def paint(gfx: Graphics2D)
+  def paint(gfx: Graphics2D): Unit =
   {
     gfx.setRenderingHints(Metrics.rendering_hints)
 
     for (node <- graphview.current_node)
       Shapes.highlight_node(gfx, graphview, node)