80  * scrolling while the viewport is obscured by another window or offscreen,
  81  * so this optimization is usually worth the performance hit when obscured.
  82  * <p>
  83  * <strong>Warning:</strong> Swing is not thread safe. For more
  84  * information see <a
  85  * href="package-summary.html#threading">Swing's Threading
  86  * Policy</a>.
  87  * <p>
  88  * <strong>Warning:</strong>
  89  * Serialized objects of this class will not be compatible with
  90  * future Swing releases. The current serialization support is
  91  * appropriate for short term storage or RMI between applications running
  92  * the same version of Swing.  As of 1.4, support for long term storage
  93  * of all JavaBeans&trade;
  94  * has been added to the <code>java.beans</code> package.
  95  * Please see {@link java.beans.XMLEncoder}.
  96  *
  97  * @author Hans Muller
  98  * @author Philip Milne
  99  * @see JScrollPane

 100  */
 101 @SuppressWarnings("serial") // Same-version serialization only
 102 public class JViewport extends JComponent implements Accessible
 103 {
 104     /**
 105      * @see #getUIClassID
 106      * @see #readObject
 107      */
 108     private static final String uiClassID = "ViewportUI";
 109 
 110     /** Property used to indicate window blitting should not be done.
 111      */
 112     static final Object EnableWindowBlit = "EnableWindowBlit";
 113 
 114     /**
 115      * True when the viewport dimensions have been determined.
 116      * The default is false.
 117      */
 118     protected boolean isViewSizeSet = false;
 119 

  80  * scrolling while the viewport is obscured by another window or offscreen,
  81  * so this optimization is usually worth the performance hit when obscured.
  82  * <p>
  83  * <strong>Warning:</strong> Swing is not thread safe. For more
  84  * information see <a
  85  * href="package-summary.html#threading">Swing's Threading
  86  * Policy</a>.
  87  * <p>
  88  * <strong>Warning:</strong>
  89  * Serialized objects of this class will not be compatible with
  90  * future Swing releases. The current serialization support is
  91  * appropriate for short term storage or RMI between applications running
  92  * the same version of Swing.  As of 1.4, support for long term storage
  93  * of all JavaBeans&trade;
  94  * has been added to the <code>java.beans</code> package.
  95  * Please see {@link java.beans.XMLEncoder}.
  96  *
  97  * @author Hans Muller
  98  * @author Philip Milne
  99  * @see JScrollPane
 100  * @since 1.2
 101  */
 102 @SuppressWarnings("serial") // Same-version serialization only
 103 public class JViewport extends JComponent implements Accessible
 104 {
 105     /**
 106      * @see #getUIClassID
 107      * @see #readObject
 108      */
 109     private static final String uiClassID = "ViewportUI";
 110 
 111     /** Property used to indicate window blitting should not be done.
 112      */
 113     static final Object EnableWindowBlit = "EnableWindowBlit";
 114 
 115     /**
 116      * True when the viewport dimensions have been determined.
 117      * The default is false.
 118      */
 119     protected boolean isViewSizeSet = false;
 120