diff --git a/packages/core/src/browser/shell/tab-bar-toolbar.spec.ts b/packages/core/src/browser/shell/tab-bar-toolbar.spec.ts
new file mode 100644
index 0000000000000..a9994cfe3de14
--- /dev/null
+++ b/packages/core/src/browser/shell/tab-bar-toolbar.spec.ts
@@ -0,0 +1,62 @@
+/********************************************************************************
+ * Copyright (C) 2019 TypeFox 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 { enableJSDOM } from '../test/jsdom';
+
+let disableJSDOM = enableJSDOM();
+import { expect } from 'chai';
+import { TabBarToolbarItem } from './tab-bar-toolbar';
+
+disableJSDOM();
+
+describe('tab-bar-toolbar', () => {
+
+    describe('comparator', () => {
+
+        before(() => {
+            disableJSDOM = enableJSDOM();
+        });
+
+        after(() => {
+            disableJSDOM();
+        });
+
+        const testMe = TabBarToolbarItem.PRIORITY_COMPARATOR;
+
+        it("should favour the 'navigation' group before everything else", () => {
+            expect(testMe({ id: 'a', command: 'a', group: 'navigation' }, { id: 'b', command: 'b', group: 'other' })).to.be.equal(-1);
+        });
+
+        it("should treat 'undefined' groups as 'navigation'", () => {
+            expect(testMe({ id: 'a', command: 'a' }, { id: 'b', command: 'b' })).to.be.equal(0);
+            expect(testMe({ id: 'a', command: 'a', group: 'navigation' }, { id: 'b', command: 'b' })).to.be.equal(0);
+            expect(testMe({ id: 'a', command: 'a' }, { id: 'b', command: 'b', group: 'navigation' })).to.be.equal(0);
+            expect(testMe({ id: 'a', command: 'a' }, { id: 'b', command: 'b', group: 'other' })).to.be.equal(-1);
+        });
+
+        it("should fall back to 'priority' if the groups are the same", () => {
+            expect(testMe({ id: 'a', command: 'a', priority: 1 }, { id: 'b', command: 'b', priority: 2 })).to.be.equal(-1);
+            expect(testMe({ id: 'a', command: 'a', group: 'navigation', priority: 1 }, { id: 'b', command: 'b', priority: 2 })).to.be.equal(-1);
+            expect(testMe({ id: 'a', command: 'a', priority: 1 }, { id: 'b', command: 'b', group: 'navigation', priority: 2 })).to.be.equal(-1);
+            expect(testMe({ id: 'a', command: 'a', priority: 1, group: 'other' }, { id: 'b', command: 'b', priority: 2 })).to.be.equal(1);
+            expect(testMe({ id: 'a', command: 'a', group: 'other', priority: 1 }, { id: 'b', command: 'b', priority: 2, group: 'other' })).to.be.equal(-1);
+            expect(testMe({ id: 'a', command: 'a', priority: 10 }, { id: 'b', command: 'b', group: 'other', priority: 2 })).to.be.equal(-1);
+            expect(testMe({ id: 'a', command: 'a', group: 'other', priority: 10 }, { id: 'b', command: 'b', group: 'other', priority: 10 })).to.be.equal(0);
+        });
+
+    });
+
+});
diff --git a/packages/core/src/browser/shell/tab-bar-toolbar.tsx b/packages/core/src/browser/shell/tab-bar-toolbar.tsx
index 8978286cc6f31..a6caeaccd1d2b 100644
--- a/packages/core/src/browser/shell/tab-bar-toolbar.tsx
+++ b/packages/core/src/browser/shell/tab-bar-toolbar.tsx
@@ -292,26 +292,18 @@ export namespace TabBarToolbarItem {
      */
     export const PRIORITY_COMPARATOR = (left: TabBarToolbarItem, right: TabBarToolbarItem) => {
         // The navigation group is special as it will always be sorted to the top/beginning of a menu.
-        if (left.group === undefined || left.group === 'navigation') {
-            return 1;
-        }
-        if (right.group === undefined || right.group === 'navigation') {
-            return -1;
-        }
-        if (left.group && right.group) {
-            if (left.group < right.group) {
-                return -1;
-            } else if (left.group > right.group) {
-                return 1;
-            } else {
-                return 0;
+        const compareGroup = (leftGroup: string | undefined = 'navigation', rightGroup: string | undefined = 'navigation') => {
+            if (leftGroup === 'navigation') {
+                return rightGroup === 'navigation' ? 0 : -1;
             }
-        }
-        if (left.group) {
-            return -1;
-        }
-        if (right.group) {
-            return 1;
+            if (rightGroup === 'navigation') {
+                return leftGroup === 'navigation' ? 0 : 1;
+            }
+            return leftGroup.localeCompare(rightGroup);
+        };
+        const result = compareGroup(left.group, right.group);
+        if (result !== 0) {
+            return result;
         }
         return (left.priority || 0) - (right.priority || 0);
     };