@Override
		public void actionPerformed( ActionEvent e ) {
			JMenuBar menuBar = (JMenuBar) e.getSource();
			JMenu menu = menuBar.getMenu( 0 );
			if( menu != null ) {
				MenuSelectionManager.defaultManager().setSelectedPath( SystemInfo.isWindows
					? new MenuElement[] { menuBar, menu }
					: new MenuElement[] { menuBar, menu, menu.getPopupMenu() } );

				FlatLaf.showMnemonics( menuBar );
			}
		}
Activates the menu bar and shows mnemonics.
On Windows, the popup of the first menu is not shown.
On other platforms, the popup of the first menu is shown.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatMenuBarUI.java