Кoрректная работа z-index

function myBtn() { 
  document.getElementById('modalW').style.display = 'block'; 
} 
 
function closee() { 
  document.getElementById('modalW').style.display = 'none'; 
} 
 
window.onclick = function(event) { 
  if (event.target == document.getElementById('modalW')) { 
    document.getElementById('modalW').style.display = 'none'; 
  } 
}

#second { 
  position: relative; 
} 
 
.meat { 
  z-index: 1; 
  position: absolute; 
  top: 96px; 
  left: 443px; 
} 
 
#second .bg { 
  position: absolute; 
  z-index: -1 
} 
 
.bg img { 
  height: 500px; 
} 
 
.menuList { 
  position: relative; 
  bottom: 13px; 
  left: 190px; 
  box-shadow: 6px 5px 2px 0 rgba(0, 0, 0, 0.06); 
  z-index: 3; 
} 
 
.menuList__right-buttons { 
  position: relative; 
  z-index: 1; 
  bottom: 50px; 
  left: 620px; 
} 
 
.menuList__right-buttons img { 
  width: 50px; 
  height: 50px; 
} 
 
#three img { 
  width: 900px; 
} 
 
.modalOne { 
  display: none; 
  position: fixed; 
  left: 0; 
  top: 0; 
  background: rgba(0, 0, 0, 0.6); 
  width: 100%; 
  height: 100%; 
  z-index: 9999; 
} 
 
.modalW { 
  margin: 4% auto; 
  width: 596px; 
  height: 482px; 
  background: #f8f9fa; 
  padding: 0px 40px 10px 41px; 
}

<section id="second"> 
  <div class="bg"> 
    <img src=https://ic.pics.livejournal.com/alkrylov/23585211/329031/329031_original.jpg> 
  </div> 
  <div class="meat"> 
    <img src=http://www.hinnawi.org.il/wp-content/uploads/Brisket.png> 
  </div> 
  <div class="menuList"> 
    <img src=http://freedesignfile.com/upload/2015/01/Modern-restaurant-menu-list-design-vectors.jpg> 
  </div> 
  <div class="menuList__right-buttons"> 
    <img src=https://i.ebayimg.com/thumbs/images/m/mdLOP8WN4_-K3ERQyPzgPvQ/s-l225.jpg> 
  </div> 
  <div id="three"> 
    <img src=http://media.womtec.com/yellow-color/yellow.jpg> 
  </div> 
</section> 
<div class="modalOne" id="modalW"> 
  <div class="modalW"> 
    <img src=http://dontforget.pro/wp-content/uploads/2014/04/modal.png> 
  </div> 
</div> 
<button onclick="myBtn()">Связаться с нами</button>