Viewport issue on slider

function isElementVisible(element) {
    ...
    var elementRect = element.node.getBoundingClientRect();
    var geoBounds = element.geometry.bounds;
    var highestBound = {
        left: Math.min(elementRect.left, geoBounds.left),
        right: Math.max(elementRect.right, geoBounds.right),
        top: Math.max(elementRect.top, geoBounds.top),
        bottom: Math.max(elementRect.bottom, geoBounds.bottom),
    }


    var elementBounds = {
        top: highestBound.top + element.geometry.height * viewFactor,
        right: highestBound.right - element.geometry.width * viewFactor,
        bottom: highestBound.bottom - element.geometry.height * viewFactor,
        left: highestBound.left + element.geometry.width * viewFactor
    };
    ....
}