the scroll arrows do not have a proper z-index and get hidden when an item that is half-shown behind them is hovered, because this item gets a higher z-index.

attached is a video of the problem

WORKAROUND:
- set z-index to the arrows:

        .RadMenu .rmBottomArrow,
        .RadMenu .rmTopArrow {
            z-index: 31 !important;
        }