Sys.Application.add_init(function () {
    let kendo = window.$telerik._kendo;
    let VirtualList = kendo.ui.VirtualList;
    // keep a reference in case you need to call the original implementation
    let originalGetElementIndex = VirtualList.fn.getElementIndex;

    VirtualList.fn.getElementIndex = function (element) {
        if (!(element instanceof kendo.jQuery)) {
            return undefined;
        }
        return parseInt(element.attr('data-offset-index'), 10);
    };
});