Fix memory-leak in FigureUtilities by disposing GC on font change

Before this change figureUtilities created a GC once for an application and then reused it to calculate font/text dimensions. Due to the recent modification of GC in SWT to store Operations, every operation on FigureUtilities that sets the font on the GC (i.e., that calls setFont()) creates a SetFontOperation in the GC. Since that GC is never disposed, these operation accumulate throughout the application lifecycle and constituted a memory leak

The current change addresses this issue by instead of crearting a single GC we create a single shell for the lifetime of the application. Whenever a different font needs to be set, the old GC is properly disposed and replaced with a new one created from the shell. This ensures that no GC objects persist indefinitely, effectively preventing the memory leak.

Author: arunjose696

org.eclipse.draw2d/src/org/eclipse/draw2d/FigureUtilities.java

@@ -32,6 +32,7 @@ public class FigureUtilities {
 
 	private static final float RGB_VALUE_MULTIPLIER = 0.6f;
 	private static GC gc;
+	private static Shell shell;
 	private static Font appliedFont;
 	private static FontMetrics metrics;
 	private static Color ghostFillColor = new Color(null, 31, 31, 31);
@@ -59,7 +60,7 @@ public static Color darker(Color color) {
 	public static FontMetrics getFontMetrics(Font f) {
 		setFont(f);
 		if (metrics == null) {
-			metrics = getGC().getFontMetrics();
+			metrics = gc.getFontMetrics();
 		}
 		return metrics;
 	}
@@ -74,14 +75,20 @@ public static FontMetrics getFontMetrics(Font f) {
 	@Deprecated
 	protected static GC getGC() {
 		if (gc == null) {
-			Shell shell = new Shell();
+			gc = new GC(getShell());
+			appliedFont = gc.getFont();
+		}
+		return gc;
+	}
+
+	private static Shell getShell() {
+		if (shell == null) {
+			shell = new Shell();
 			InternalDraw2dUtils.configureForAutoscalingMode(shell, event -> {
 				// ignored
 			});
-			gc = new GC(shell);
-			appliedFont = gc.getFont();
 		}
-		return gc;
+		return shell;
 	}
 
 	/**
@@ -95,7 +102,7 @@ protected static GC getGC() {
 	 */
 	protected static org.eclipse.swt.graphics.Point getTextDimension(String s, Font f) {
 		setFont(f);
-		return getGC().textExtent(s);
+		return gc.textExtent(s);
 	}
 
 	/**
@@ -123,7 +130,7 @@ public static IFigure getRoot(IFigure figure) {
 	 */
 	protected static org.eclipse.swt.graphics.Point getStringDimension(String s, Font f) {
 		setFont(f);
-		return getGC().stringExtent(s);
+		return gc.stringExtent(s);
 	}
 
 	/**
@@ -341,10 +348,14 @@ public static void paintEtchedBorder(Graphics g, Rectangle r) {
 	 * @since 2.0
 	 */
 	protected static void setFont(Font f) {
-		if (appliedFont == f || (f != null && f.equals(appliedFont))) {
+		if ((appliedFont == null && f == null && gc != null) || (f != null && f.equals(appliedFont))) {
 			return;
 		}
-		getGC().setFont(f);
+		if (gc != null && !gc.isDisposed()) {
+			gc.dispose();
+		}
+		gc = new GC(getShell());
+		gc.setFont(f);
 		appliedFont = f;
 		metrics = null;
 	}