Hi,
Thank for the reply. Apologies for being unclear. What I should've said is this long div is actually a gallery containing a set of list items which are thumbnails. On left button hover the horizontal gallery will slide left non-stop until you reach the end of the ul. I also have a right button to scroll right but I didn't put it in my codepen example. My problem is when you hover on the left button the gallery ul will only slide '-=200px' once.
Here is a new codepen. - http://codepen.io/anon/pen/zqEGeg
The setInterval codepen example from my first comment is super close to what I want to achieve. But the gallery ul doesn't stop animating instantly when you stop hovering on the button.