This gets the height of any element relative to what is visible on screen

getVisibleHeight.js

function getVisibleHeight(el) {
    var elementHeight = el.offsetHeight;
    var windowHeight = window.innerHeight;
    var rect = el.getBoundingClientRect();
    var topValue = rect.top;
    var bottomValue = rect.bottom;
    return Math.max(0, topValue > 0 ? Math.min(elementHeight, windowHeight - topValue) : (bottomValue < windowHeight ? bottomValue : windowHeight));
}