Add static method to configure overlay scrolling for FigureCanvas

This allows the user to specify whether new instances of FigureCanvas
should use overlay scrolling on not. This is disabled by default, due to
performance issues on Linux/GTK3.

As a result, the scrollbar is shown as a separate entity and no longer
part of the client area. This setting only affects operating systems
supporting overlay scrolling. This method requires SWT 3.126 or newer.

eclipse#430

CHANGELOG.md

@@ -1,3 +1,10 @@
+# GEF Classic 3.20.0 (Eclipse 2024-06)
+
+## Draw2d
+ - Overlay scrolling is disabled by default for FigureCanvas. Clients can
+   restore the original behavior by calling FigureCanvas.setScrollbarsMode()
+   prior to creating the object.
+
 # GEF Classic 3.19.0 (Eclipse 2024-03)
 
 ## Draw2d

org.eclipse.draw2d/src/org/eclipse/draw2d/FigureCanvas.java

@@ -13,6 +13,9 @@
 package org.eclipse.draw2d;
 
 import java.beans.PropertyChangeListener;
+import java.lang.reflect.Method;
+import java.util.logging.Level;
+import java.util.logging.Logger;
 
 import org.eclipse.swt.SWT;
 import org.eclipse.swt.events.SelectionAdapter;
@@ -21,6 +24,7 @@
 import org.eclipse.swt.widgets.Canvas;
 import org.eclipse.swt.widgets.Composite;
 import org.eclipse.swt.widgets.Control;
+import org.eclipse.swt.widgets.Scrollable;
 
 import org.eclipse.draw2d.geometry.Dimension;
 import org.eclipse.draw2d.geometry.Point;
@@ -49,6 +53,7 @@
  */
 public class FigureCanvas extends Canvas {
 
+	private static final Logger LOGGER = Logger.getLogger(FigureCanvas.class.getSimpleName());
 	private static final int ACCEPTED_STYLES = SWT.RIGHT_TO_LEFT | SWT.LEFT_TO_RIGHT | SWT.V_SCROLL | SWT.H_SCROLL
 			| SWT.NO_BACKGROUND | SWT.NO_REDRAW_RESIZE | SWT.DOUBLE_BUFFERED | SWT.BORDER;
 
@@ -73,6 +78,16 @@ public class FigureCanvas extends Canvas {
 	/** Always show scrollbar */
 	public static int ALWAYS = 2;
 
+	/**
+	 * Static property specifying whether new instances of this class should be
+	 * created with overlay scrolling enabled. Possible values are {@link SWT#NONE}
+	 * or {@link SWT#SCROLLBAR_OVERLAY}. This value is set to {@link SWT#NONE} by
+	 * default, due to performance issues on Linux/GTK3.
+	 *
+	 * @see <a href="https://github.com/eclipse/gef-classic/issues/430">here</a>
+	 */
+	private static int SCROLLBARS_MODE = SWT.NONE;
+
 	private int vBarVisibility = AUTOMATIC;
 	private int hBarVisibility = AUTOMATIC;
 	private Viewport viewport;
@@ -155,6 +170,7 @@ public FigureCanvas(Composite parent, LightweightSystem lws) {
 	 */
 	public FigureCanvas(int style, Composite parent, LightweightSystem lws) {
 		super(parent, checkStyle(style));
+		adjustScrollbarsMode();
 		getHorizontalBar().setVisible(false);
 		getVerticalBar().setVisible(false);
 		this.lws = lws;
@@ -255,6 +271,25 @@ public Viewport getViewport() {
 		return viewport;
 	}
 
+	/**
+	 * Calls {@link Scrollable#setScrollbarsMode(int)} if executed with SWT 3.126.
+	 *
+	 * @since 3.2
+	 */
+	private void adjustScrollbarsMode() {
+		// TODO Check version of SWT binary after PR has been merged:
+		// https://github.com/eclipse-platform/eclipse.platform.swt/pull/1188
+		if (SWT.getVersion() >= 4965 && getScrollbarsMode() != SCROLLBARS_MODE) {
+			try {
+				// Invoke method via reflection to remain compatible with older SWT versions
+				Method setScrollbarsMode = Scrollable.class.getMethod("setScrollbarsMode", Integer.class); //$NON-NLS-1$
+				setScrollbarsMode.invoke(this, SCROLLBARS_MODE);
+			} catch (ReflectiveOperationException e) {
+				LOGGER.log(Level.SEVERE, e.getMessage(), e);
+			}
+		}
+	}
+
 	/**
 	 * Adds listeners for scrolling.
 	 */
@@ -552,6 +587,29 @@ public void setViewport(Viewport vp) {
 		hookViewport();
 	}
 
+	/**
+	 * Global property to enable overlay scrolling for all figure canvas when
+	 * {@link SWT#SCROLLBAR_OVERLAY} and to disable overlay scrolling when
+	 * {@link SWT#NONE} is given. The default value is {@link SWT#NONE}.<br>
+	 * Calling this method has no effect on already created instances of this
+	 * class.<br>
+	 * Calling this method has no effect if overlay scrolling is not supported by
+	 * the operating system.
+	 *
+	 * @param mode One of {@link SWT#NONE}, {@link SWT#SCROLLBAR_OVERLAY}.
+	 * @since 3.16
+	 * @throws IllegalArgumentException if the given {@code mode} isn't one of
+	 *                                  {@link SWT#NONE}
+	 *                                  {@link SWT#SCROLLBAR_OVERLAY}.
+	 * @see #SCROLLBARS_MODE
+	 */
+	public static void setScrollbarsMode(int mode) throws IllegalArgumentException {
+		if (mode != SWT.NONE && mode != SWT.SCROLLBAR_OVERLAY) {
+			throw new IllegalArgumentException("Scrollbars mode must be either SWT.SCROLLBAR_OVERLAY or SWT.NONE"); //$NON-NLS-1$
+		}
+		SCROLLBARS_MODE = mode;
+	}
+
 	private static int verifyScrollBarOffset(RangeModel model, int value) {
 		value = Math.max(model.getMinimum(), value);
 		return Math.min(model.getMaximum() - model.getExtent(), value);