fix: add ViewerComparator compatibility workaround for Outline view

CommonViewerComparator was introduced in Eclipse 4.39 and so Outline
can't use this class on older Eclipse versions. Let use old
CommonViewerSorter for compatibility with older Eclipse releases.

See eclipse-platform/eclipse.platform.ui#3621
Fixes #1515

Author: iloveeclipse

org.eclipse.lsp4e/src/org/eclipse/lsp4e/outline/CNFOutlinePage.java

@@ -40,6 +40,7 @@
 import org.eclipse.jface.viewers.TreePath;
 import org.eclipse.jface.viewers.TreeSelection;
 import org.eclipse.jface.viewers.TreeViewer;
+import org.eclipse.jface.viewers.ViewerComparator;
 import org.eclipse.lsp4e.LSPEclipseUtils;
 import org.eclipse.lsp4e.LanguageServerPlugin;
 import org.eclipse.lsp4e.LanguageServerWrapper;
@@ -53,7 +54,6 @@
 import org.eclipse.swt.widgets.Control;
 import org.eclipse.ui.IActionBars;
 import org.eclipse.ui.navigator.CommonViewer;
-import org.eclipse.ui.navigator.CommonViewerComparator;
 import org.eclipse.ui.texteditor.ITextEditor;
 import org.eclipse.ui.views.contentoutline.IContentOutlinePage;
 
@@ -75,6 +75,9 @@ public class CNFOutlinePage implements IContentOutlinePage, ILabelProviderListen
 
 	private final LanguageServerWrapper wrapper;
 
+	@Nullable
+	private static Boolean canUseCommonViewerComparator;
+
 	public CNFOutlinePage(LanguageServerWrapper wrapper, @Nullable ITextEditor textEditor) {
 		preferences = InstanceScope.INSTANCE.getNode(LanguageServerPlugin.PLUGIN_ID);
 		preferences.addPreferenceChangeListener(this);
@@ -90,7 +93,7 @@ public void createControl(final Composite parent) {
 		if (document != null) {
 			outlineViewer.setInput(new OutlineViewerInput(document, wrapper, textEditor));
 		}
-		outlineViewer.setComparator(new CommonViewerComparator());
+		outlineViewer.setComparator(createComparator());
 		outlineViewer.getLabelProvider().addListener(this);
 		final var textEditor = this.textEditor;
 		if (textEditor != null) {
@@ -124,6 +127,34 @@ public void createControl(final Composite parent) {
 		}
 	}
 
+	/**
+	 * Try to be compatible with older Eclipse versions (before 4.39) where
+	 * CommonViewerComparator is not available.
+	 *
+	 * @return comparator for the outline
+	 */
+	private static ViewerComparator createComparator() {
+		if (checkIfCommonViewerComparatorAvailable()) {
+			return Comparators.createCommonViewerComparator();
+		}
+		// Can be removed if Eclipse 4.38 is no longer supported
+		return Comparators.createCommonViewerSorter();
+	}
+
+	private static boolean checkIfCommonViewerComparatorAvailable() {
+		if(canUseCommonViewerComparator != null) {
+			return canUseCommonViewerComparator.booleanValue();
+		}
+		try {
+			Class.forName("org.eclipse.ui.navigator.CommonViewerComparator"); //$NON-NLS-1$
+			canUseCommonViewerComparator = Boolean.TRUE;
+			return true;
+		} catch (ClassNotFoundException e) {
+			canUseCommonViewerComparator = Boolean.FALSE;
+			return false;
+		}
+	}
+
 	/**
 	 * Returns the range of the given selection and null otherwise.
 	 *

org.eclipse.lsp4e/src/org/eclipse/lsp4e/outline/Comparators.java

@@ -0,0 +1,41 @@
+/*******************************************************************************
+ * Copyright (c) 2026 Andrey Loskutov <loskutov@gmx.de>.
+ * This program and the accompanying materials are made
+ * available under the terms of the Eclipse Public License 2.0
+ * which is available at https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *  Andrey Loskutov <loskutov@gmx.de> - initial implementation
+ *******************************************************************************/
+package org.eclipse.lsp4e.outline;
+
+import org.eclipse.jface.viewers.ViewerComparator;
+import org.eclipse.ui.navigator.CommonViewerComparator;
+import org.eclipse.ui.navigator.CommonViewerSorter;
+
+/**
+ * Support different Eclipse platform versions providing different ViewerComparator implementations
+ */
+public class Comparators {
+	/**
+	 * Compatible with modern Eclipse versions (4.39 and later) where
+	 * CommonViewerComparator is available.
+	 *
+	 * @return comparator for the outline
+	 */
+	public static ViewerComparator createCommonViewerComparator() {
+		return new CommonViewerComparator();
+	}
+
+	/**
+	 * Compatible with older Eclipse versions (before 4.39) where
+	 * CommonViewerComparator is not available.
+	 *
+	 * @return comparator for the outline
+	 */
+	public static ViewerComparator createCommonViewerSorter() {
+		return new CommonViewerSorter();
+	}
+}