title: offset().top is not working as expected in if parent element is fixed
body: <p>I am using <code>position : fixed</code> for a div for stick that div on top of the page, but when i use <code>offset().top</code> for its child value, it gives different values on scrolling. i tried like :</p>
  
  <p>CSS :</p>
  
  <pre><code>.parent{
      position : fixed;
      top : 0px;
  }
  .child{
      height : 20px;
      margin-top : 10px;
  }
  </code></pre>
  
  <p>JS : </p>
  
  <pre><code>console.log($(".child").offset().top);
  </code></pre>
  
  <p>the above JS code give different results when i scrolling the page.</p>