HTML2Canvas does not render full div, only what is visible on screen?

html2canvas(htmlSource, {scrollY: -window.scrollY}).then(function(canvas) {
var img = canvas.toDataURL();
window.open(img);
});

Credit: https://stackoverflow.com/questions/36213275/html2canvas-does-not-render-full-div-only-what-is-visible-on-screen/59841384#59841384