Detecting a system window overlaying an iframe : <p>After looking at <a href="https://www.youtube.com/watch?v=TzPq5_kCfow" rel="noreferrer">this youtube video</a> I was curious how some of the features shown, can be implemented with JS.</p>

<p>One of my major questions is how can one detect another system window (like the word window, shown in the video) on the iframe.</p>

<p>On <a href="https://www.youtube.com/watch?v=RSWhvNdjAAE&amp;feature=youtu.be" rel="noreferrer">another video</a> there is a hint suggesting that the technic is based on the fact that browsers optimize rendering for elements that are out of the view.  </p>

<p>I couldn't tap into what are the exact methods/properties that are used.</p>

<p>What are your thoughts?  </p>
