Avoid concurrency computation of the items to display in notebook too…

…lbar (jupyterlab#15954)

packages/ui-components/src/components/toolbar.tsx

@@ -486,7 +486,7 @@ export class ReactiveToolbar extends Toolbar<Widget> {
       // Invokes resizing to ensure correct display of items after an addition, only
       // if the toolbar is rendered.
       if (this.isVisible) {
-        void this._onResize();
+        void this._resizer.invoke();
       }
     }
     return status;