Add flag to enable double-buffering when doing paint operations

By using the `org.eclipse.draw2d.LightweightSystem.useDoubleBuffering`,
clients can enable a special functionality, that first paints all UI
updates to an internal buffer, before it is painted to the screen.

The hope of behavior is to increase the perceived performance of an
application by detaching rendering from the display.

eclipse-gef#430

CHANGELOG.md

@@ -1,4 +1,7 @@
-# GEF Classic 3.20.0 
+# GEF Classic 3.20.0 (Eclipse 2024-03)
+
+## Draw2d
+ - _Experimental:_ Clients can set the  `org.eclipse.draw2d.LightweightSystem.useDoubleBuffering` environment variable to enable double buffering while performing paint operations.
 
 ## GEF
 - Clients can now configure the accessible steps of tools through methods `accGetStep()`, `accStepIncrement` and `accStepReset` from `org.eclipse.gef.tools.AbstractTool`.

org.eclipse.draw2d/.settings/.api_filters

@@ -40,6 +40,14 @@
             </message_arguments>
         </filter>
     </resource>
+    <resource path="src/org/eclipse/draw2d/LightweightSystem.java" type="org.eclipse.draw2d.LightweightSystem">
+        <filter id="336658481">
+            <message_arguments>
+                <message_argument value="org.eclipse.draw2d.LightweightSystem"/>
+                <message_argument value="KEY_USE_DOUBLE_BUFFERING"/>
+            </message_arguments>
+        </filter>
+    </resource>
     <resource path="src/org/eclipse/draw2d/Orientable.java" type="org.eclipse.draw2d.Orientable">
         <filter id="576720909">
             <message_arguments>

org.eclipse.draw2d/src/org/eclipse/draw2d/LightweightSystem.java

@@ -34,6 +34,7 @@
 import org.eclipse.swt.graphics.Color;
 import org.eclipse.swt.graphics.Font;
 import org.eclipse.swt.graphics.GC;
+import org.eclipse.swt.graphics.Image;
 import org.eclipse.swt.widgets.Canvas;
 import org.eclipse.swt.widgets.Event;
 import org.eclipse.swt.widgets.Listener;
@@ -54,13 +55,23 @@
  * </ol>
  */
 public class LightweightSystem {
+	/**
+	 * Indicates whether double-buffering should be used when painting the figures.
+	 * This is an experimental feature that might be removed in a future release.
+	 *
+	 * @since 3.16
+	 * @noreference This field is not intended to be referenced by clients.
+	 */
+	public static final String KEY_USE_DOUBLE_BUFFERING = "org.eclipse.draw2d.LightweightSystem.useDoubleBuffering"; //$NON-NLS-1$
+	private static final boolean USE_DOUBLE_BUFFERING = Boolean.valueOf(System.getProperty(KEY_USE_DOUBLE_BUFFERING));
 
 	private Canvas canvas;
 	IFigure contents;
 	private IFigure root;
 	private EventDispatcher dispatcher;
 	private UpdateManager manager = new DeferredUpdateManager();
 	private int ignoreResize;
+	private Image bufferedImage;
 
 	/**
 	 * Constructs a LightweightSystem on Canvas <i>c</i>.
@@ -117,6 +128,7 @@ protected void controlResized() {
 		if (ignoreResize > 0) {
 			return;
 		}
+		bufferedImage = null;
 		Rectangle r = new Rectangle(canvas.getClientArea());
 		r.setLocation(0, 0);
 		root.setBounds(r);
@@ -199,7 +211,21 @@ EventHandler internalCreateEventHandler() {
 	 * @since 2.0
 	 */
 	public void paint(GC gc) {
-		getUpdateManager().paint(gc);
+		if (USE_DOUBLE_BUFFERING) {
+			if (bufferedImage == null) {
+				Rectangle bounds = getRootFigure().getBounds();
+				bufferedImage = new Image(null, bounds.width, bounds.height);
+			}
+			GC bufferedGC = new GC(bufferedImage);
+			try {
+				getUpdateManager().paint(bufferedGC);
+				gc.drawImage(bufferedImage, 0, 0);
+			} finally {
+				bufferedGC.dispose();
+			}
+		} else {
+			getUpdateManager().paint(gc);
+		}
 	}
 
 	/**