Capybara & Poltergeist: get element's position or size

Getting element's position or size is not that simple in Capybara and Poltergeist. You will need to evaluate some JS to archive what you want.

getting position:

pos = page.driver.evaluate_script <<-EOS
  function() {
    var ele  = document.getElementById('someId');
    var rect = ele.getBoundingClientRect();
    return [rect.left, rect.top];
  }();
EOS

getting size:

size = page.driver.evaluate_script <<-EOS
  function() {
    var ele  = document.getElementById('someId');
    var rect = ele.getBoundingClientRect();
    return [rect.width, rect.height];
  }();
EOS

These samples could be easily combined if you want both pos and size.