JavaScript, событие onmouseover на элементе под элементом

var movdiv = document.getElementById('movob'); 
movdiv.onmousedown = function(curevent) { 
  moveAt(curevent); 
 
  function moveAt(curevent) { 
    movex = curevent.clientX; 
    movey = curevent.clientY; 
    movdiv.style.left = (movex - 75) + 'px'; 
    movdiv.style.top = (movey - 75) + 'px'; 
  } 
 
  document.onmousemove = function(curevent) { 
    moveAt(curevent); 
  } 
 
  movdiv.onmouseup = function(curevent) { 
    document.onmousemove = null; 
  } 
} 
 
var clidiv = document.getElementById('cliob'); 
clidiv.onmouseover = function(event2) { 
  console.log('over!'); 
}

<div id="back" style="position: absolute; background-color: yellow; width: 1000px; height: 600px; left: 50px; right: 50px;"> 
  <div id="cliob" style="position: absolute; background-color: grey; width: 100px; height: 100px; left: 250px; top: 250px;"></div> 
  <div id="movob" style="position: absolute; background-color: blue; width: 100px; height: 100px; left: 50px; top: 50px; z-index: 50;"></div> 
</div>