title: How to detect browser's protocol handlers?
body: <p>I have created a custom URL protocol handler.</p>
  
  <p><code>http://</code></p>
  
  <p><code>mailto://</code></p>
  
  <p><strong><code>custom://</code></strong></p>
  
  <p>I have registered a WinForms application to respond accordingly. This all works great.</p>
  
  <p>But I would like to be able to gracefully handle the case where the user doesn't have the custom URL protocol handler installed, yet.</p>
  
  <p>In order to be able to do this I need to be able to detect the browser's registered protocol handlers, I would assume from JavaScript. But I have been unable to find a way to poll for the information. I am hoping to find a solution to this problem.</p>
  
  <p>Thanks for any ideas you might be able to share.</p>