Move MenuBar index check to an overridable method

packages/widgets/src/menubar.ts

@@ -140,13 +140,8 @@ export class MenuBar extends Widget {
    * If the menu cannot be activated, the index will be set to `-1`.
    */
   set activeIndex(value: number) {
-    // Adjust the value for an out of range index.
-    if (value < 0 || value >= this._menus.length) {
-      value = -1;
-    }
-
-    // An empty menu cannot be active
-    if (value > -1 && this._menus[value].items.length === 0) {
+    // Adjust the value for an invalid index.
+    if (!this.isValidIndex(value)) {
       value = -1;
     }
 
@@ -162,6 +157,20 @@ export class MenuBar extends Widget {
     this.update();
   }
 
+  /**
+   * Before setting a new index, this method is used to validate it.
+   *
+   * By default it checks whether the index is within menu range
+   * and whether the corresponding menu has at least one item.
+   */
+  protected isValidIndex(index: number): boolean {
+    return (
+      index >= 0 &&
+      index < this._menus.length &&
+      this._menus[index].items.length > 0
+    );
+  }
+
   /**
    * A read-only array of the menus in the menu bar.
    */

packages/widgets/tests/src/menubar.spec.ts

@@ -294,6 +294,29 @@ describe('@lumino/widgets', () => {
         expect(bar.activeIndex).to.equal(-1);
         bar.dispose();
       });
+
+      it('should allow adopting the validation check', () => {
+        class AllowEmptyMenusBar extends MenuBar {
+          protected override isValidIndex(index: number): boolean {
+            return index >= 0 && index < this.menus.length;
+          }
+        }
+        const bar = new AllowEmptyMenusBar();
+        let emptyMenu = new Menu({ commands });
+        bar.insertMenu(1, emptyMenu);
+
+        // check that empty menu can be active
+        bar.activeIndex = 1;
+        expect(bar.activeIndex).to.equal(1);
+
+        // other indices should still be disallowed
+        bar.activeIndex = -1;
+        expect(bar.activeIndex).to.equal(-1);
+        bar.activeIndex = 2;
+        expect(bar.activeIndex).to.equal(-1);
+
+        bar.dispose();
+      });
     });
 
     describe('#menus', () => {

review/api/widgets.api.md

@@ -789,6 +789,7 @@ export class MenuBar extends Widget {
     dispose(): void;
     handleEvent(event: Event): void;
     insertMenu(index: number, menu: Menu, update?: boolean): void;
+    protected isValidIndex(index: number): boolean;
     get menus(): ReadonlyArray<Menu>;
     protected onActivateRequest(msg: Message): void;
     protected onAfterDetach(msg: Message): void;