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;
@@ -54,6 +55,7 @@
 import org.eclipse.ui.IActionBars;
 import org.eclipse.ui.navigator.CommonViewer;
 import org.eclipse.ui.navigator.CommonViewerComparator;
+import org.eclipse.ui.navigator.CommonViewerSorter;
 import org.eclipse.ui.texteditor.ITextEditor;
 import org.eclipse.ui.views.contentoutline.IContentOutlinePage;
 
@@ -75,6 +77,9 @@ 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 +95,7 @@ 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 +129,63 @@ 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 Eclipse439PlusComparator.create();
+		}
+		return EclipseBefore439Comparator.create();
+	}
+
+	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;
+		}
+	}
+
+	/**
+	 * Compatible with modern Eclipse versions (4.39 and later) where
+	 * CommonViewerComparator is available.
+	 * <p>
+	 * This is extracted to an extra class to avoid classloading issues on older Eclipse versions where
+	 * the CommonViewerComparator class is not available. If we try to load a class referencing
+	 * CommonViewerComparator on an older Eclipse version, it will result in a ClassNotFoundException.
+	 *
+	 * @return comparator for the outline
+	 */
+	private static final class Eclipse439PlusComparator {
+		static ViewerComparator create() {
+			return new CommonViewerComparator();
+		}
+	}
+
+	/**
+	 * Compatible with older Eclipse versions (before 4.39) where
+	 * CommonViewerComparator is not available.
+	 * <p>
+	 * This is extracted to an extra class to avoid classloading issues
+	 *
+	 * @return comparator for the outline
+	 */
+	private static final class EclipseBefore439Comparator {
+		static ViewerComparator create() {
+			return new CommonViewerSorter();
+		}
+	}
+
 	/**
 	 * Returns the range of the given selection and null otherwise.
 	 *