To only render part of a page you need to set the clipRect attribute for the page and then render it.
var clipRect = document.querySelector(selector).getBoundingClientRect();
page.clipRect = {
top: clipRect.top,
left: clipRect.left,
width: clipRect.width,
height: clipRect.height
};
page.render('capture.png');
I don't understand the second part of your question. Phantom.js is headless meaning that there is no actual display that a user is looking at.
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