Although I haven't tested this myself, it looks like your arrow button is not going under your VRule, it's just being cut off because its x position places it outside the boundaries of the item renderer.
To do something like this you may need to have the left-list slightly overlap the right-list, and have the item renderer with the arrow in it have a transparent background. That's getting a bit complicated though. You may be better off just working around it, having the arrow move slightly to the right inside its item renderer rather than actually over top of the right list
For your renderer, use a VBox or other container that supports the clipContent property and set it to false, then set horizontalScrollPolicy of the renderer to false, then the arrow button will be visible outside its container.
If this post answers your question or helps, please mark it as such.
Its still not helping me
I put off all scrollpolicy of each container and set clipcontent=false but
still not working