According to CSS – Overflow: Scroll; – Always show vertical scroll bar?: OSx Lion hides scrollbars while not in use to make it seem more “slick”, but at the same time the issue you addressed comes up: people sometimes cannot see whether a div has a scroll feature or not.
CSS fix:
::-webkit-scrollbar {
-webkit-appearance: none;
width: 7px;
}
::-webkit-scrollbar-thumb {
border-radius: 4px;
background-color: rgba(0,0,0,.5);
-webkit-box-shadow: 0 0 1px rgba(255,255,255,.5);
}