preferences: fix PreferenceProvider.merge method (eclipse-theia#12126)

Merge properties of type array when merging preferences objects.

packages/core/src/browser/preferences/preference-provider.spec.ts

@@ -0,0 +1,36 @@
+// *****************************************************************************
+// Copyright (C) 2023 EclipseSource and others.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Eclipse Public License v. 2.0 which is available at
+// http://www.eclipse.org/legal/epl-2.0.
+//
+// This Source Code may also be made available under the following Secondary
+// Licenses when the conditions for such availability set forth in the Eclipse
+// Public License v. 2.0 are satisfied: GNU General Public License, version 2
+// with the GNU Classpath Exception which is available at
+// https://www.gnu.org/software/classpath/license.html.
+//
+// SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
+// *****************************************************************************
+
+import { PreferenceProvider } from './preference-provider';
+const { expect } = require('chai');
+
+describe('PreferenceProvider', () => {
+    it('should preserve extra source fields on merge', () => {
+        const result = PreferenceProvider.merge({ 'configurations': [], 'compounds': [] }, { 'configurations': [] });
+        expect(result).deep.equals({ 'configurations': [], 'compounds': [] });
+    });
+    it('should preserve extra target fields on merge', () => {
+        const result = PreferenceProvider.merge({ 'configurations': [] }, { 'configurations': [], 'compounds': [] });
+        expect(result).deep.equals({ 'configurations': [], 'compounds': [] });
+    });
+    it('should merge array values', () => {
+        const result = PreferenceProvider.merge(
+            { 'configurations': [{ 'name': 'test1', 'request': 'launch' }], 'compounds': [] },
+            { 'configurations': [{ 'name': 'test2' }] }
+        );
+        expect(result).deep.equals({ 'configurations': [{ 'name': 'test1', 'request': 'launch' }, { 'name': 'test2' }], 'compounds': [] });
+    });
+});

packages/core/src/browser/preferences/preference-provider.ts

@@ -235,6 +235,9 @@ export abstract class PreferenceProvider implements Disposable {
                 if (JSONExt.isObject(source[key]) && JSONExt.isObject(value)) {
                     this.merge(source[key], value);
                     continue;
+                } else if (JSONExt.isArray(source[key]) && JSONExt.isArray(value)) {
+                    source[key] = [...JSONExt.deepCopy(source[key] as any), ...JSONExt.deepCopy(value)];
+                    continue;
                 }
             }
             source[key] = JSONExt.deepCopy(value);