title: detect visibility status of an element
body: <p>Hi I would like to detect whether an element is visible or hidden. I thought about this </p>
  
  <pre><code>$("#mobile_navbar:visible").change(function() {
     console.log("detected")
  });
  </code></pre>
  
  <p>but this does not seem to work...? Any ideas how to do this? Basically I what to check whether $("#mobile_navbar").is( ":visible" ) has changed... but I want that a function is called whenever that happens, lick the .click() function which gets triggered if an element is clicked on.<br>
  thanks
  carl</p>