Snippet to reproduce exception when calling getExpandedTreePaths()

This snippet shows how to cause an AssertionFailedException when calling
getExpandedTreePaths() on a virtual tree.

When clicking the "refresh" button for the first time, everything works
fine.

On the second time, the following error is logged:

> !MESSAGE Invalid index value 2 >= 2

On the third time, an AssertionFailedException is thrown.

See:
#2491

examples/org.eclipse.jface.snippets/Eclipse JFace Snippets/org/eclipse/jface/snippets/viewers/Snippet070VirtualViewerWithExpandedState.java

@@ -0,0 +1,182 @@
+/*******************************************************************************
+ * Copyright (c) 2024 DSA GmbH and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     DSA GmbH - initial API and implementation
+ *******************************************************************************/
+
+package org.eclipse.jface.snippets.viewers;
+
+import java.net.MalformedURLException;
+import java.net.URI;
+
+import org.eclipse.core.runtime.Platform;
+import org.eclipse.jface.resource.ImageDescriptor;
+import org.eclipse.jface.resource.JFaceResources;
+import org.eclipse.jface.resource.ResourceManager;
+import org.eclipse.jface.viewers.ColumnLabelProvider;
+import org.eclipse.jface.viewers.ILazyTreeContentProvider;
+import org.eclipse.jface.viewers.TreeNode;
+import org.eclipse.jface.viewers.TreePath;
+import org.eclipse.jface.viewers.TreeViewer;
+import org.eclipse.swt.SWT;
+import org.eclipse.swt.events.SelectionListener;
+import org.eclipse.swt.graphics.Image;
+import org.eclipse.swt.layout.BorderData;
+import org.eclipse.swt.layout.BorderLayout;
+import org.eclipse.swt.widgets.Shell;
+import org.eclipse.swt.widgets.ToolBar;
+import org.eclipse.swt.widgets.ToolItem;
+
+/**
+ * A simple example to illustrate how to preserve the expand/collapse state when
+ * using a virtual viewer.
+ */
+public class Snippet070VirtualViewerWithExpandedState {
+	public static class SimpleNode extends TreeNode {
+		public SimpleNode(Object value) {
+			super(value);
+		}
+
+		public int getChildCount() {
+			SimpleNode[] children = getChildren();
+			return children == null ? 0 : children.length;
+		}
+
+		public void setParentChildren(SimpleNode... children) {
+			for (TreeNode child : children) {
+				child.setParent(this);
+			}
+			setChildren(children);
+		}
+
+		@Override
+		public SimpleNode[] getChildren() {
+			return (SimpleNode[]) super.getChildren();
+		}
+
+		@Override
+		public String toString() {
+			return "[%s], %s".formatted(getClass().getSimpleName(), getValue());
+		}
+	}
+
+	public static class LazyContentProvider implements ILazyTreeContentProvider {
+		private final TreeViewer viewer;
+
+		public LazyContentProvider(TreeViewer viewer) {
+			this.viewer = viewer;
+		}
+		@Override
+		public void updateElement(Object parent, int index) {
+			SimpleNode[] children = ((SimpleNode) parent).getChildren();
+			if (children == null) {
+				// viewer.setHasChildren(parent, false);
+			} else if (children.length > index) {
+				SimpleNode child = children[index];
+				viewer.replace(parent, index, child);
+				viewer.setChildCount(child, child.getChildCount());
+			} else {
+				Platform.getLog(getClass()).warn("Invalid index value %d >= %d".formatted(index, children.length));
+			}
+		}
+
+		@Override
+		public void updateChildCount(Object element, int currentChildCount) {
+			int newChildCount = ((SimpleNode) element).getChildCount();
+			if (currentChildCount != newChildCount) {
+				viewer.setChildCount(element, newChildCount);
+			}
+		}
+
+		@Override
+		public Object getParent(Object element) {
+			return ((SimpleNode) element).getParent();
+		}
+	}
+
+	static SimpleNode root;
+	static SimpleNode node0;
+	static SimpleNode node1;
+	static SimpleNode node2;
+	static SimpleNode node3;
+	static SimpleNode node4;
+	static SimpleNode node5;
+	static SimpleNode node6;
+	static SimpleNode node7;
+	static SimpleNode node8;
+	public static boolean flip = false;
+
+	public static void flip() {
+		flip = !flip;
+		if (flip) {
+			node0.setParentChildren(node1, node2, node8);
+		} else {
+			node0.setParentChildren(node2, node8);
+		}
+	}
+
+	public static void main(String[] args) throws MalformedURLException {
+		root = new SimpleNode("Invisible");
+		node0 = new SimpleNode(0);
+		node1 = new SimpleNode(1);
+		node2 = new SimpleNode(2);
+		node3 = new SimpleNode(3);
+		node4 = new SimpleNode(4);
+		node5 = new SimpleNode(5);
+		node6 = new SimpleNode(6);
+		node7 = new SimpleNode(7);
+		node8 = new SimpleNode(8);
+
+		root.setParentChildren(node0);
+		node0.setParentChildren(node2, node8);
+		node2.setParentChildren(node3, node4, node5);
+		node5.setParentChildren(node6, node7);
+
+		Shell shell = new Shell();
+		shell.setSize(500, 155);
+		shell.setLayout(new BorderLayout());
+
+		TreeViewer viewer = new TreeViewer(shell, SWT.MULTI | SWT.H_SCROLL | SWT.V_SCROLL | SWT.BORDER | SWT.VIRTUAL);
+		viewer.setContentProvider(new LazyContentProvider(viewer));
+		viewer.setLabelProvider(new ColumnLabelProvider());
+		viewer.getTree().setLayoutData(new BorderData(SWT.CENTER));
+		viewer.setUseHashlookup(true);
+		viewer.setInput(root);
+		viewer.expandAll();
+
+		ToolBar toolBar = new ToolBar(shell, SWT.NONE);
+		toolBar.setLayoutData(new BorderData(SWT.TOP));
+
+		ResourceManager resourceManager = JFaceResources.managerFor(shell);
+		ImageDescriptor imageDescriptor = ImageDescriptor
+				.createFromURL(URI.create("platform:/plugin/org.eclipse.pde.ui/icons/elcl16/refresh.png").toURL());
+		Image image = resourceManager.create(imageDescriptor);
+		ToolItem toolItem = new ToolItem(toolBar, SWT.PUSH);
+		toolItem.setImage(image);
+		toolItem.addSelectionListener(SelectionListener.widgetSelectedAdapter(event -> {
+			flip();
+
+			TreePath[] treePaths = viewer.getExpandedTreePaths();
+			viewer.setInput(root);
+			viewer.setExpandedTreePaths(treePaths);
+
+			// This refresh is important, as it recalculates all tree items. Because the
+			// viewer doesn't fit in the shell, some of those items remain virtual, which
+			// then cause an exception, the next time this listener is executed.
+			viewer.refresh();
+		}));
+
+		shell.open();
+		while (!shell.isDisposed()) {
+			shell.getDisplay().readAndDispatch();
+		}
+	}
+}