If you visit this website: http://demdous.com/examples/html/serendipity/ and resite it to mobile size, then open the menu and dont close it. Now resize to fullscreen and you will see that mobile menu is still there. How to fix that?
its working fine for me, dont' know what you are looking at, or something try to check it in diff browsers, also, try to see if there is a browser extension that is messing it up.