             updateDisplayListVirtual();
         else
             updateDisplayListReal();
-    } 
+    }
+    
+    /**
+     *  @private 
+     *  Convenience function for subclasses that invalidates the
+     *  target's size and displayList so that both layout's <code>measure()</code>
+     *  and <code>updateDisplayList</code> methods get called.
+     * 
+     *  <p>Typically a layout invalidates the target's size and display list so that
+     *  it gets a chance to recalculate the target's default size and also size and
+     *  position the target's elements. For example changing the <code>gap</code>
+     *  property on a <code>VerticalLayout</code> will internally call this method
+     *  to ensure that the elements are re-arranged with the new setting and the
+     *  target's default size is recomputed.</p> 
+     */
+    private function invalidateTargetSizeAndDisplayList():void
+    {
+        var g:GroupBase = target;
+        if (!g)
+            return;
+
+        g.invalidateSize();
+        g.invalidateDisplayList();
+    }
 }
 }
 

	 *  Note that the layout element may only be partially in view.
	 * 
	 *  @see lastIndexInView
	 *  @see inView
	 *  
	 *  @langversion 3.0
	 *  @playerversion Flash 10

     *  Note that the row may only be partially in view.
     * 
     *  @see firstIndexInView
     *  @see fractionOfElementInView
	 *  
	 *  @langversion 3.0
	 *  @playerversion Flash 10
