Check if div is scrolled to bottom, leave alone if not

Question

I recently developed a live online chat room. I have everything working and secure but I want the div that the messages are stored in to autoscroll to the bottom. I managed to do that with this Javascript:

window.setInterval(function() {
  var elem = document.getElementById('messages');
  elem.scrollTop = elem.scrollHeight;
}, 100);

But with this, the user can't scroll upwards at all, it is always pushing them back down to the bottom of the div. What I want to happen is if the user is already at the bottom of the div, then when a new message appears it scrolls them to the bottom of the div. BUT if the user is NOT at the bottom of the div then leave them alone until they scroll to the bottom.

Here's the jQuery that updates the div:

window.onload = function(){

            setInterval(function(){
                $.ajax({
                    url: "get.php",
                    type: "GET",
                    success: function(output){
                       $("div#messages").html("<p>" + output + "</p>");

                    }
                });
            }, 500);

        }

Then the div itself is just empty from the start: <div id="messages"></div>

Here's the CSS if you need it:

#messages {
    width: 700px;
    height: 250px;
    border: 2px solid black;
    overflow: auto;
    -moz-border-radius: 15px;
    border-radius: 15px;
}

Both jQuery and Javascript will work for this, I'm not restricted to using one or the other.

Answer 1

scrollTop, at maximum scroll, will be equal to the scrollHeight minus offsetHeight of the element. This is because the offsetHeight is included in the scrollHeight.

Setting scrollTop to scrollHeight works because the browser automatically adjusts the variable to the maximum scroll allowed (scrollHeight - offsetHeight).

Here's the logic you should use:

if (elem.scrollTop >= (elem.scrollHeight - elem.offsetHeight)) {
    elem.scrollTop = elem.scrollHeight;
}

Just slap the if statement around where you're assigning elem.scrollTop to elem.scrollHeight