How sendEvent in slimerjs doesn't work

sendEvent.js

var page = require('webpage').create();
page.open("http://slimerjs.org", function(status) {
    if (status == 'success') {
        page.render('out.jpg', {quality: '100'});
    }
})

window.setTimeout(function() {
    var pair = page.evaluate(function(){
        var button = document.getElementById("download");
        var x = button.getBoundingClientRect().x;
        var y = button.getBoundingClientRect().y;
        return [x, y];
    });

    var x = pair[0]; var y = pair[1];
    page.sendEvent('click', x+2, y+2, 'left');

    // second screenshot (nothing has changed!)
    window.setTimeout(function() {
        page.render('out2.jpg', {quality: '100'});
    }, 5000);
} , 1000);