src/vs/base/browser/ui/menu/menubar.ts/MenuBar/updateMnemonicVisibility
class MenuBar:	private updateMnemonicVisibility(visible: boolean): void {
		if (this.menuCache) {
			this.menuCache.forEach(menuBarMenu => {
				if (menuBarMenu.titleElement.children.length) {
					let child = menuBarMenu.titleElement.children.item(0) as HTMLElement;
					if (child) {
						child.style.textDecoration = (this.options.alwaysOnMnemonics || visible) ? 'underline' : '';
					}
				}
			});
		}
	}