You need to set a specific height on the “comments” div to make sure it knows exactly when to scroll. If there’s not enough content to fill up that container beyond the specified height, the scrollbar might appear with overflow:scroll but it will be disabled. If you want the scrollbar to appear only when it’s actually needed, you’ll want to use overflow:auto as the CSS rule. By setting the height of the child container and not the parent, the parent can grow as necessary.
In your example, the position:absolute on the parent container is not required to obtain the solution; however, you might be including that for some other reason.