stomaks,
возможно вы хотели так ...
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
#menu {
display: flex;
overflow: auto;
}
.item {
min-width: 50%;
}
</style>
</head>
<body>
<div id="content">
<div id="menu">
<div class="item">1</div>
<div class="item">2</div>
<div class="item">3</div>
<div class="item">4</div>
<div class="item">5</div>
<div class="item">6</div>
<div class="item">7</div>
<div class="item">8</div>
<div class="item">9</div>
<div class="item">10</div>
</div>
</div>
<script>
window.addEventListener('DOMContentLoaded', function() {
var menu = document.querySelector('#menu'),
node = menu.querySelectorAll('.item');
[].reduce.call(node, function(scrollLeft,item) {
var scroll = scrollLeft + item.offsetWidth;
item.addEventListener('click', function() {
menu.scrollLeft = menu.scrollLeft == scrollLeft ? scroll : scrollLeft
});
return scroll
},0);
});
</script>
</body>
</html>