Закрепленное меню и соседний div

var node = document.getElementById('menu_sticky'), 
  nodeOffs = node.offsetTop; 
window.addEventListener('scroll', function() { 
  var scrollPos = (window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0); 
  if (scrollPos > nodeOffs) { 
    node.classList.add('_stickied'); 
  } else { 
    node.classList.remove('_stickied'); 
  } 
});

body { 
  padding: 300px 0 0 0; 
  margin:0; 
  height: 200vh; 
} 
.menu._sticky { 
  height: 50px; 
  width:100%; 
  background-color:red; 
} 
 
.menu._sticky._stickied { 
  position: fixed; 
  top: 0; 
  left: 0; 
}

<nav class="menu _header _sticky" id="menu_sticky"></nav>